Description Title of Invention: METHOD FOR MANUFACTURING LIGHT GUIDE DEVICE, LIGHT GUIDE DEVICE, AND VIRTUAL IMAGE DISPLAY APPARATUS Technical Field [0001] The present invention relates to a method for manufacturing a light guide device that introduces image light so as to present an image formed by an image display element to an observer, and more particularly, a method for manufacturing a light guide device suitable for a head-mounted display mounted on a head portion of the observer, a light guide device, and a virtual image display apparatus using the light guide device. Background Art [0002] Various optical systems have been proposed as an optical system assembled with a virtual image display apparatus such as a head-mounted display (hereinafter referred to as HMD) mounted on a head portion of an observer. Particularly, various prisms have been proposed as light guide prisms arranged in front of an eye (for example, refer to PTL 1, 2, and the like). [0003] With regard to the virtual image display apparatus such as the HMD, it is desired to accelerate a reduction in size and weight and to achieve a wide angle of view without deteriorating image quality. Also, in a state where only image light is viewed with an entire field of vision of the observer being covered, the observer cannot determine a state of an outside world to be nervous. Meanwhile, with see-through view in which the outside world and the image are shown in an overlapped manner, various novel ap plications of virtual reality are generated. Accordingly, display expressing the image light overlapped without hindering the field of vision of the outside world is desired. [0004] Allowing for the above-described situation, a light guide device is configured, for example, by a penetration-type prism arranged in a see-through manner in front of the eye of the observer with a plurality of free-form surfaces such that the observer can have a better feeling of mounting similar to a shape of glasses while a form of an ap pearance can be excellent. In a case of the light guide device applied to the virtual image display device with a see-through function, an inclined semi-transmissive surface is disposed on one end of a block-shaped light guide prism so as to overlap, for example, external light with the image light. In this case, in order to complement the inclination of the semi-transmissive surface and cover the front of the eye uniformly, another block-shaped light-transmitting member (opposing prism) that forms a pair is required to be bonded. During the bonding, acceptable see-through performance and image formation are not ensured if, for example, bubbles enter a bonding site and a distortion is generated on a reflection surface. Accordingly, it is important to appro priately control, for example, a flow direction of the adhesive during adhesion for the bonding and to have an excellent adhesive filling state. Citation List Patent Literature [0005] [PTL 1] JP-A-1 1-149003 [PTL 2] JP-A- 10-268 114 Summary of Invention Technical Problem [0006] However, in a case where the bonding site is formed to be a free-form surface, it is not always easy to control the adhesive to be filled in a desired state by applying the adhesive to be the joint to the free-form surface and bond the free-form surface. [0007] Even in a case where the see-through function is not provided, the inclined reflective surface is disposed so as to, for example, take out the image light. In this case, there are appearance and strength problems, and there is a case where block-shaped members that form a pair are bonded at a site disposed on the reflective surface and, there is a concern over a distortion generated with respect to such reflective surface as in the case where the see-through function is provided. Solution to Problem [0008] An advantage of some aspects of the invention is to provide a method for manu facturing a light guide device that can be assembled with a virtual image display apparatus, which has a shape similar to a shape of glasses to have an excellent form of an appearance, has high manufacturing workability in a bonding part between the block-shaped light guide prism and the opposing prism, and allows an optical function to be ensured, the light guide device, and a virtual image display apparatus using the light guide device. [0009] An aspect of the invention is directed to a method for manufacturing (a) a light guide device including (al) a light guide prism that includes a mirror layer which performs reflection of image light propagated inward on a first bonding surface which is a light guide side bonding surface, (a2) an opposing prism that includes a second bonding surface which is arranged to oppose the first bonding surface and is a bonding surface on an opposite side corresponding to the first bonding surface, and is bonded with the light guide prism by bonding the first bonding surface and the second bonding surface with each other across the mirror layer, (a3) and a joint that is formed by an adhesive which is filled in an adhesion area between the first bonding surface and the second bonding surface, (b) the first bonding surface of the light guide prism being a free-form surface and (c) the second bonding surface of the opposing prism being a free-form surface having substantially the same shape as the first bonding surface, the method including (d) a preparing step in which the first bonding surface and the second bonding surface are caused to oppose each other, and the light guide prism and the opposing prism are arranged to be capable of approaching each other in a specific offset direction in which an inclination angle with respect to one end side of the first bonding surface and an inclination angle with respect to the other end side have the same orientation but different sizes; (e) an adhesive applying step in which the adhesive is applied to at least one surface of the first bonding surface and the second bonding surface that oppose each other in the preparing step; (f an adhering step in which the second bonding surface and the first bonding surface are caused to approach each other in the offset direction such that the adhesive applied during the adhesive applying is interposed so as to be filled between the light guide prism and the opposing prism; and (g) a bonding step in which the adhesive that is filled between the first bonding surface and the second bonding surface during the adhering step is cured such that the joint which bonds the light guide prism and the opposing prism with each other is formed. Herein, the inclination angle of the offset direction with respect to the light guide side bonding surface means an angle formed by a directional vector showing the offset direction with respect to a plane defining the direction of the light guide side bonding surface at each point of the light guide side bonding surface, which is, for example, a tangent plane at the point. Also, the one inclination angle and the other in clination angle having the same orientation means that, for example, a sign of an inner product with regard to a normal vector of the plane specifying each inclination angle and the directional vector showing the offset direction is the same between the one in clination angle and the other inclination angle. According to the method for manufacturing the light guide device, the light guide prism and the opposing prism are caused to approach each other from the specific offset direction and the first bonding surface and the second bonding surface are bonded with each other such that the light guide prism and the opposing prism are connected to each other. In this case, by using a difference in the inclination angle in the offset direction with respect to the first bonding surface, sizes of clearances (adhesion areas) formed between both of the surfaces during the adhesion can be different depending on positions on the first bonding surface even in a case where the first bonding surface and the second bonding surface which have the free-form surfaces unlikely to be aligned are adhered with each other. Because of the clearance difference, a filling process such as a flow direction of the adhesive can be controlled to be simple and a state where optical accuracy is high can be maintained while improving workability during the manufacturing of the joint by filling of the adhesive. In addition, the first bonding surface and the second bonding surface where the mirror layer which performs the reflection of the image light while forming the joint can be the free-form surfaces and the other surfaces that form the appearance such as the surfaces contributing to the light guiding can be the free-form surfaces such that an eye-front part of the light guide device can be close in shape to a shape of glasses and a form of the appearance is excellent. [001 1] In a specific aspect of the invention, in the method for manufacturing the light guide device described above, the first bonding surface of the light guide prism has a convex surface shape, the second bonding surface of the opposing prism has a concave surface shape, and the offset direction in the preparing step is a direction in which the in clination angle with respect to the one end side of the first bonding surface is smaller than the inclination angle with respect to the other end side. In this case, during the approach of the second bonding surface and the first bonding surface in the adhering, the clearance on the one end side of the first bonding surface can be narrower than the clearance on the other side in an earlier stage. In this manner, the filling process such as the flow direction of the adhesive can be controlled into a desired state. [0012] In another specific aspect of the invention, in the method for manufacturing the light guide device described above, the adhesive in the adhesive applying step is applied to the one end side of the first bonding surface, and the adhesive in the adhering step is caused to flow from a side of the first bonding surface where the inclination angle with respect to the offset direction is small to a side where the inclination angle is large. In this case, the adhesive applied to the one end side of the first bonding surface can flow toward the other end side of the first bonding surface while performing the filling of the adhesive. [0013] In still another specific aspect of the invention, in the method for manufacturing the light guide device described above, the light guide prism has a projection-shape first bonding rib, the opposing prism has a projection- shaped second bonding rib that cor responds to the first bonding rib, and, in the adhering step, the light guide prism and the opposing prism are adhered with each other through a sliding movement along the offset direction with the first bonding rib and the second bonding rib being references for holding. In this case, a relative positional relationship between the light guide prism and the opposing prism can be precise and a state where control accuracy of the flow of the adhesive in the adhering is high can be maintained by performing the adhesion by the sliding movement with the first and second bonding ribs being the references for holding. [0014] In yet another specific aspect of the invention, in the method for manufacturing the light guide device described above, the light guide prism includes first and second faces that are arranged to oppose each other across the first bonding surface, and the free-form surface which introduces the image light from an image display element on an inner surface side while reflecting the light, and the light guide prism includes the first bonding rib in a state of being along at least a part of a lateral end portion disposed out of an area of the first bonding surface of the first and second faces. In this case, the first and second faces are shaped to include the free-form surfaces and the appearance and a mounting state of the virtual image display apparatus assembled with the light guide device are similar to glasses such that a curved surface shape along the face can be maintained with regard to the first and second faces forming the eye-front part and a part of the appearance while the guiding of the image light can be ensured and visibility can be approximately zero during observation of an outside world image in a case of see-through. Moreover, the first bonding rib is disposed along outer circum ferences of the first and second faces such that an effect on the appearance of the first bonding rib of the assembled virtual image display apparatus can be suppressed. [0015] In still yet another specific aspect of the invention, the method for manufacturing the light guide device described above further includes a molding step in which the light guide prism and the opposing prism are respectively formed by injection molding, in which, in the molding step, the first bonding rib and the second bonding rib are in tegrally formed in a state of being respectively along at least a part of the lateral end portion of the light guide prism and a lateral end portion of the opposing prism. In this case, the light guide prism and the opposing prism that respectively include the first bonding rib and the second bonding rib can be formed with relative simplicity and high accuracy. [0016] In further another specific aspect of the invention, in the method for manufacturing the light guide device described above, the first and second bonding ribs respectively have first and second flat plate portions that extend along the lateral end portion, and the first and second flat plate portions are respectively disposed along a parting line of a molding die which performs the injection molding in the molding step to define the offset direction. In this case, the high-accuracy rib can be manufactured in a simple manner. [0017] In still further another specific aspect of the invention, in the method for manu facturing the light guide device described above, in the adhering step, the light guide prism and the opposing prism are caused to approach and to be adhered with each other in a state where a surface of the first flat plate portion of the first bonding rib and a surface of the second flat plate portion of the second bonding rib are arranged on the same plane. In this case, for example, parallel directions on the same plane can be defined as the offset direction such that accurate adhesion and bonding processing can be performed. [0018] In yet further specific aspect of the invention, in the method for manufacturing the light guide device described above, a shape of the free-form surface with regard to the first bonding surface of the light guide prism and the second bonding surface of the opposing prism is determined by performing a parameter adjustment according to control of a flow direction of the adhesive based on the following equation in which x, y, and z are coordinate values in polar coordinates, c is a curvature, A is each co efficient parameter, and r0 is a normalized aperture. [Math.l In this case, with respect to the flow direction of the adhesive, controlling to accelerate the flow in various directions can be more reliably performed and, for example, the controlling of the flow with regard to the direction other than the direction corre sponding to the offset direction can be made. [0019] In still yet further another specific aspect of the invention, in the method for manu facturing the light guide device described above, the light guide side bonding surface of the light guide prism and the opposing side bonding surface of the opposing prism are bonded with each other in a shape in which a thickness of an end portion of the adhesion area forming the joint on a side close to the observer is larger than a thickness of the end portion on a far side with respect to a differential between distances in which the joint is interposed. In this case, the joint that is formed can be unlikely to pose inconvenience to a field of vision even when the amount of the adhesive to be the joint is increased sufficiently to spread throughout an adhesion range in the bonding process since, of the part to be the joint, the width of the joint is increased for the observer and the side that is unlikely to be blurred is increased in size. [0020] In a further specific aspect of the invention, in the method for manufacturing the light guide device described above, the light guide prism allows penetration of external light while guiding the image light, and the mirror layer of the light guide prism is a half mirror layer that performs partial reflection of the image light and partial passing of the external light. In this case, the image light and the external light can be observed at the same time via the half mirror layer. [0021] In a still further specific aspect of the invention, in the method for manufacturing the light guide device described above, the opposing prism is an auxiliary prism that is bonded with and integrally fixed to the light guide prism and assists in a see-through function of the light guide prism. In this case, the field of vision of the outside world can be excellent via the light guide device. [0022] The method for manufacturing the virtual image display apparatus uses the abovedescribed method for manufacturing the light guide device, and thus a state where the accuracy of the joint is high can be maintained and the other surfaces that form the ap pearance such as the surfaces contributing to the light guiding can be the free-form surfaces such that the shape can be similar to a shape of glasses and the form of the ap pearance is excellent. [0023] Another aspect of the invention is directed to a light guide device including (a) a light guide prism that includes a mirror layer which performs reflection of image light propagated inward on a first bonding surface which is a light guide side bonding surface, (b) an opposing prism that includes a second bonding surface which is arranged to oppose the first bonding surface and is a bonding surface on an opposite side corresponding to the first bonding surface, and is bonded with the light guide prism by bonding the first bonding surface and the second bonding surface with each other across the mirror layer, and (c) a joint that is formed by an adhesive which is filled in an adhesion area between the first bonding surface and the second bonding surface, in which (d) the first bonding surface of the light guide prism is a free-form surface, (e) the second bonding surface of the opposing prism is a free-form surface having substantially the same shape as the first bonding surface, (f the light guide prism has a projection- shaped first bonding rib, (g) the opposing prism has a projection- shaped second bonding rib that corresponds to the first bonding rib, and (h) in the light guide prism and the opposing prism, the first bonding rib and the second bonding rib extend along a specific offset direction in which an inclination angle with respect to one end side of the first bonding surface and an inclination angle with respect to the other end side are different from each other. [0024] During the manufacturing of the light guide device, the approach and bonding between first bonding surface and the second bonding surface along the specific offset direction in which the first bonding rib and the second bonding rib extend is facilitated. During the bonding, a difference in the inclination angle in the offset direction with respect to the first bonding surface is used such that sizes of the clearances (adhesion areas) which are formed between both of the surfaces during the adhesion can also be different depending on the positions on the first bonding surface. Accordingly, even in a case where the first bonding surface and the second bonding surface have the freeform surfaces that are unlikely to be aligned, a filling process such as a flow direction of the adhesive can be controlled such that the filling state in the joint formed from the adhesive can be excellent while a state where the accuracy is high can be maintained in the light guide device. In addition, the first bonding surface and the second bonding surface where the mirror layer which performs the reflection of the image light while forming the joint can be the free-form surfaces and the other surfaces that form the ap pearance such as the surfaces contributing to the light guiding can be the free-form surfaces such that an eye-front part of the light guide device can be close in shape to a shape of glasses and a form of the appearance is excellent. [0025] In a specific aspect of the invention, in the light guide device described above, the first bonding rib and the second bonding rib are respectively integrally formed with the light guide prism and the opposing prism by injection molding in a state of respectively being along at least a part of a lateral end portion of the light guide prism and a lateral end portion of the opposing prism. [0026] In another specific aspect of the invention, in the light guide device described above, the first and second bonding ribs respectively have first and second flat plate portions that extend along the lateral end portion, and the first and second flat plate portions are respectively disposed along a parting line of a molding die which performs the injection molding to define the offset direction. [0027] In still another specific aspect of the invention, in the light guide device described above, a shape of the free-form surface with regard to the first bonding surface of the light guide prism and the second bonding surface of the opposing prism is determined by performing a parameter adjustment according to control of a flow direction of the adhesive based on the following equation in which x, y, and z are coordinate values in polar coordinates, c is a curvature, A is each coefficient parameter, and r0 is a normalized aperture. [0028] Still another aspect of the invention is directed to a first virtual image display apparatus including an image display element that generates image light, and a light guide device that introduces image light from the image display element on an inner surface side while reflecting the light. [0029] The above-described virtual image display apparatus uses the above-described light guide device, and thus a state where the accuracy of the joint is high can be maintained and the other surfaces that form the appearance such as the surfaces contributing to the light guiding can be the free-form surfaces such that the shape can be similar to a shape of glasses and the form of the appearance is excellent. [0030] Yet another aspect of the invention is directed to a second virtual image display apparatus (a) that allows an observer to recognize an image as a virtual image, including (b) an image display element that generates image light, (c) a light guide prism that allows the image to be recognized toward an eye of the observer, and (d) an opposing prism that includes a mirror layer that reflects light of the image from the light guide prism toward the observer, and constitutes a see-through portion with a bonding surface of the light guide prism across the mirror layer, in which (e) in the light guide prism, the bonding surface of the light guide prism is a free-form surface, (f) in the opposing prism, a bonding surface of the opposing prism is a free-form surface having substantially the same shape as the light guide side bonding surface, (g) the light guide prism has a projection- shaped first bonding rib, (h) the opposing prism has a projection- shaped second bonding rib that corresponds to the first bonding rib, and (i) in the light guide prism and the opposing prism, the first bonding rib and the second bonding rib extend along a specific offset direction in which an inclination angle with respect to one end side of the bonding surface of the light guide prism and an inclination angle with respect to the other end side are different from each other. [0031] In the virtual image display apparatus, during the manufacturing of the light guide device, the approach and bonding between the bonding surface of the light guide prism and the bonding surface of the opposing prism along the specific offset direction in which the first bonding rib and the second bonding rib extend is facilitated such that the filling state in the joint formed from the adhesive can be excellent while a state where the accuracy is high can be maintained. Also, the other surfaces that form the appearance such as the surfaces contributing to the light guiding can be the free-form surfaces such that the shape can be similar to a shape of glasses and the form of the ap pearance is excellent. Brief Description of Drawings [0032] [fig.lA]Fig. 1A is a perspective view illustrating an appearance and a structure of a light guide device of a first embodiment. [fig.lB]Fig. IB is a perspective view of the light guide device viewed from another direction than in Fig. 1A. [fig.2](A) in Fig. 2 is a view showing a state where a light guide prism and an opposing prism are bonded with each other, (B) and (C) are schematic views showing a clearance of a bonding site, (D) is a schematic view showing an extension direction of an adhesive, (E) is a schematic view showing a modification example of an adhesion method, and (F) is a schematic view showing a light guide device of a com parative example. [fig.3]Fig. 3 is a side sectional view of a molding die illustrating prism molding by injection molding using the molding die. [fig.4]Fig. 4 is a side view illustrating the bonding between the light guide prism and the opposing prism. [fig.5]Fig. 5 is a cross-sectional view showing an installation state of the prism in Fig. 4. [fig.6]Fig. 6 is a view illustrating an inclination angle with respect to an offset direction of a first bonding surface of the light guide prism and a second bonding surface of the opposing prism. [fig.7A]Fig. 7A is a view showing a condition of the adhesive in an early stage of adhesion. [fig.7B]Fig. 7B is a view showing a condition of the adhesive in an intermediate stage of the adhesion. [fig.7C]Fig. 7C is a view showing a condition of the adhesive in a final stage of the adhesion. [fig.8]Fig. 8 is a perspective view illustrating an appearance of a virtual image display apparatus of a second embodiment incorporating the light guide device. [fig.9A]Fig. 9A is an overall perspective view showing the virtual image display apparatus. [fig.9B]Fig. 9B is a perspective view showing an internal structure of the virtual image display apparatus from which an exterior member and a protector are removed. [fig.lO]Fig. 10 is a perspective view showing a state where the exterior member is removed so as to illustrate a structure of a first display device of the virtual image display apparatus. [fig. 11A]Fig. 11A is an exploded perspective view illustrating a method for fixing a prism and a projection lens of the virtual image display apparatus to a frame [fig. 1lB]Fig. 1IB is an exploded perspective view illustrating a method for fixing a prism and a projection lens of the virtual image display apparatus to a frame. [fig.l2]Fig. 12 is a cross-sectional view of a plane of symmetry with respect to a top and a bottom of the first display device constituting the virtual image display apparatus. Description of Embodiments [0033] First Embodiment Hereinafter, a light guide device and the like of a first embodiment according to the invention will be described in detail referring to the accompanying drawings. [0034] A light guide device 20 shown in Figs. 1A and IB has a light guide prism 10 that is arranged on a projection lens 30 (refer to Fig. 1A) side, a projection lens 30 being another member associated with image formation or an image formation main body portion, and an opposing prism 50 that is a light transmitting member which is arranged on an opposite side from the projection lens 30 across the light guide prism 10, and is integrated by bonding the light guide prism 10 and the opposing prism 50 with each other. The light guide device 20 guides image light from the projection lens 30 by using the light guide prism 10, emits the light from an inner side where an eye or a face of a wearer is present, and transmits external light via the light guide prism 10 and the opposing prism 50 to the inner side where the eye of the wearer is present. [0035] The light guide device 20 is a member formed of resin that has an intricately curved three-dimensional shape and, in terms of appearance, has an eye cover unit EC that is a drop- shaped and flat ellipsoid covering a front of the eye of the wearer, and a polyhedral root portion RP that extends in an elongated manner from the eye cover unit EC. The light guide device 20 has a first face SSI on a front side far from the eye or the face of the wearer, that is, an outer side, and a second face SS2 on a back side close to the face of the wearer, that is, the inner side, which are surfaces fulfilling an optical function and covering the front of the eye to form the appearance. [0036] A lateral end portion SP that forms a front-view outline of the light guide device 20 is disposed between the first face SSI and the second face SS2, and a projecting rib 60 is disposed along a part of the lateral end portion SP. Although described in detail later, the rib 60 is disposed to correspond to top and bottom sides of the light guide device 20 viewed from the front and, particularly, a rib part on the top side is divided into a bonding rib 10h and a frame mounting rib 10k. [0037] The light guide prism 10 of the light guide device 20 has an eye-front formation portion AP, a link portion BP, and a connection portion CP. The eye-front formation portion AP is a swelled part that covers the front of the eye as a part of the eye cover unit EC of the light guide prism 10, and forms a face part of the first face SSI and the second face SS2 whose outline has an elliptical shape with the opposing prism 50. The link portion BP is disposed on a root portion RP side of the light guide prism 10, and has the frame mounting rib 10k that allows mounting onto a frame, which is another member, as a part of the rib 60. The connection portion CP is disposed on the root portion RP side, and is used to connect the light guide device 20 with the projection lens 30. In other words, the connection portion CP is a member that allows a relative alignment of the first and second faces SSI and SS2 with respect to the projection lens 30, and is a fitting portion that allows a posture alignment through locking using fitting between these and the projection lens 30. [0038] As described above, the opposing prism 50 of the light guide device 20 is bonded with the eye-front formation portion AP of the light guide prism 10 to form the e l liptical eye cover unit EC. In other words, the opposing prism 50 has surfaces corre sponding to surfaces to be the first face SSI and the second face SS2 of the light guide prism 10 so as to form the first face SSI and the second face SS2 in conjunction with the light guide prism 10. [0039] Hereinafter, the light guide prism 10 of the light guide device 20, which is a part a s sociated with the optical function of performing the guiding of the image light, will be described. The light guide prism 10 is a member that shows high optical transmission property in a visible range, is integrally formed by injection molding, and is formed of a thermoplastic resin material and, in terms of function, can be considered to be divided into a first prism part 11 on a light emission side more apart from a not-shown image display element, and a second prism part 12 on a light incident side closer to the not-shown image display element. [0040] The first prism part 11 has a first surface SI 1, a second surface S12, and a third surface S13 as surfaces having the optical function, and the second prism part 12 has a fourth surface S14, a fifth surface S15, and a sixth surface S16 as surfaces having the optical function. The first surface SI 1 and the fourth surface S14 are adjacent to each other, and the third surface S13 and the fifth surface S15 are adjacent to each other. The second surface S12 is arranged between the first surface SI 1 and the third surface S13, and the sixth surface S16 is arranged between the fourth surface S14 and the fifth surface S15. [0041] In the light guide prism 10, each of the first surface SI 1, the second surface S12, the third surface S13, the fourth surface S14, the fifth surface S15, and the sixth surface S16 is a free-form surface and is associated with imaging. [0042] Hereinafter, an optical path of the light guiding by the light guide prism 10 will be described briefly. The image light from the projection lens 30, which is another member, is first incident on the sixth surface S16 of the second prism part 12 of the light guide prism 10, is reflected by the fifth surface S15, and is further reflected by the fourth surface S14. Then, the image light is incident on the third surface S13 of the first prism part 11 to be totally reflected, and is incident on the first surface SI 1 to be totally reflected. The image light that is totally reflected by the first surface S11 is incident on the second surface S12, is partially reflected, and is incident on the first surface SI 1 again to pass therethrough. Although described in detail later, the second surface S12 has a half mirror layer 15 as a mirror layer that realizes see-through, and reflects the image light while partially transmitting the image light. The image light that passes through the first surface SI 1 allows an observer to observe an image. The first surface SI 1 and the third surface S13 are arranged in front of an eye EY and are formed into a concave surface shape with respect to the observer, and visibility is ap proximately zero when the external light passing through the first surface SI 1 and the third surface S13 is seen. In other words, in the light guide prism 10, the third surface S13 and the like as the first face SSI and the first surface SI 1 and the like as the second face SS2 opposing this are free-form surfaces, and thus the guiding of the image light is allowed and the external light can be transmitted such that the visibility is approximately zero. [0043] Hereinafter, the opposing prism 50 of the light guide device 20 will be described. As described above, the opposing prism 50 constitutes the light guide device 20 that is a single optical component integrally fixed with the light guide prism 10, and is a prism (auxiliary prism) assisting in a see-through function of the light guide prism 10. The opposing prism 50 shows high optical transmission property in the visible range, and a main body part of the opposing prism 50 is formed of a thermoplastic resin material with substantially the same refractive index as the light guide prism 10. [0044] The opposing prism 50 is a light transmitting member that has a first transmitting surface S51, a second transmitting surface S52, and a third transmitting surface S53 as side surfaces having the optical function. Herein, the second transmitting surface S52 is arranged between the first transmitting surface S51 and the third transmitting surface S53. The first transmitting surface S51 is on a curved surface that is extended from the first surface SI 1 of the light guide prism 10. From a different perspective, the first transmitting surface S51 and the first surface SI 1 form a face of the second face SS2 whose outline has an elliptical shape, particularly of the second face SS2, in front of the eye. The second transmitting surface S52 is a curved surface that is integrated by a joint JN which is formed by filling the second surface S12 with an adhesive. In other words, the second transmitting surface S52 and the second surface S12 of the light guide prism 10 are integrated with each other by bonding, and thus have shapes of sub stantially the same curvature. The third transmitting surface S53 is on a curved surface that is extended from the third surface S13 of the light guide prism 10. From a different perspective, the third transmitting surface S53 and the third surface S13 form a face of the first face SSI whose outline has an elliptical shape, particularly of the first face SSI, in front of the eye. Also, in the above case, a state where the visibility is ap proximately zero when the passing external light is seen is maintained even for the first transmitting surface S51 and the second transmitting surface S52 that are respectively extended from the first surface SI 1 and the third surface S13. In other words, the external light can be transmitted such that the visibility is approximately zero in not only the light guide prism 10 but also the opposing prism 50 and a part where both of the prisms are overlapped and bonded with each other. [0045] The opposing prism 50 has a rod-shaped dripping guide unit 18 on a tip side opposing the root portion RP side. After the opposing prism 50 is bonded to the light guide prism 10, a hard coat layer 27 is applied to a face with the light guide prism 10. In this case, during the film formation of the hard coat layer 27, the dripping guide unit 18 suppresses pooling of a coating solution to be the hard coat layer 27 and reduces a coating irregularity. [0046] Herein, the second surface S12 of the light guide prism 10 that is a site of bonding with the opposing prism 50 has a configuration using a half mirror so as to realize the see-through view in which an outside world and the image are shown in an overlapped manner. Specifically, the second surface S12 is a face of the light guide prism 10, and the half mirror layer 15 is incidental to the face. The half mirror layer 15 is a light reflection film RM (that is, a semi-transmissive reflective film) that has optical transmission property. The half mirror layer (semi-transmissive reflective film) 15 is formed on a partial area PA that is a part of the second surface S12. The half mirror layer 15 is formed by forming a metal reflective film and a dielectric multilayer film, and a reflectance of the half mirror layer 15 with respect to an image light GL is from 10% to 50% in an assumed incident angle range of the image light GL from a viewpoint of facilitating a see-through observation with regard to the external light passing through the opposing prism 50. Specifically, the reflectance of the half mirror layer 15 of the embodiment with respect to the image light GL is set to be, for example, 20%, and a transmittance with respect to the image light GL is set to be, for example, 80%. [0047] In the light guide prism 10, the second surface S12 is also a first bonding surface AS which is a surface bonded with the opposing prism 50 with the joint JN being in terposed, that is, a light guide side bonding surface. Corresponding thereto, in the opposing prism 50, the second transmitting surface S52 is a second bonding surface BS which is a surface bonded with the light guide prism 10 with the joint JN being in terposed, that is, an opposing side bonding surface. The first bonding surface AS and the second bonding surface BS are free-form surfaces, and are curved surfaces having substantially the same shape at the bonding site where these are bonded with each other. [0048] In a case where the second surface S12 is considered as the first bonding surface AS and the second transmitting surface S52 is considered as the second bonding surface BS, an entire area of the second surface S12 and an entire area of the second transmitting surface S52 are equivalent to an adhesion surface during the bonding. In other words, the joint JN in a state where the light guide prism 10 and the opposing prism 50 are bonded with each other is formed and the light guide device 20 is formed when the adhesive is applied to an adhesion area on the adhesion surface to be filled between the light guide prism 10 and the opposing prism 50 and then the adhesive is cured. Details with respect to the bonding will be described in a manufacturing process. [0049] Hereinafter, the rib 60 that is disposed over a substantially entire circumference of the light guide device 20 will be described. The rib 60 is a member formed of resin that is disposed as a part of and to be incidental to the light guide prism 10 and the opposing prism 50 and surrounds a circumferential edge of the light guide device 20. The rib 60 has the frame mounting rib 10k that has a mounting portion lOg which allows the mounting onto the frame, which is another member used to assemble a virtual image display apparatus, and bonding ribs 10h and lOp that are respectively formed on the top side and the bottom side to extend along the first face SSI and the second face SS2 and are positioning references in bonding the light guide prism 10 and the opposing prism 50 with each other. Herein, the bonding rib on an upper end side is the bonding rib 10h and the bonding rib on a lower end side is the bonding rib lOp. [0050] Of the above-described bonding ribs 10h and lOp, the bonding rib 10h on the upper end side has a flat-plated shaped flat plate portion FB 1 (first flat plate portion). Similarly, the bonding rib lOp on the lower end side has a flat plate portion FB2 (second flat plate portion). When the flat plate portions FB1 and FB2 have planar parts having high accuracy, the bonding ribs 10h and lOp can be the positioning references during the bonding between the light guide prism 10 and the opposing prism 50. For example, the bonding rib 10h on the upper end side is configured to have a first bonding rib Rl that is incidental to the light guide prism 10 and a second bonding rib R2 that is incidental to the opposing prism 50. The light guide prism 10 and the opposing prism 50 are adhered to each other through a sliding movement along an offset direction DD with the first bonding rib Rl and the second bonding rib being references for holding. Herein, as described above, the rib 60, that is, the first and second bonding ribs Rl and R2 are arranged along the lateral end portion SP that is disposed out of an area of the first bonding surface AS. The bonding rib 10h on the upper end side that has the flat plate portion FB1 (first flat plate portion) is inserted into a groove- shaped limiting unit disposed in the frame as a stopper in a loose-fit state with a slight clearance during the mounting onto the above-described and not-shown frame, and thus functions to limit a relative displacement with respect to the frame. The bonding rib 10h on the upper end side has gate portions GP1 and GP2 that are formed to be incidental to the flat plate portion FB 1. The gate portions GP1 and GP2 are equivalent to resin inlets that are used when the light guide prism 10 and the opposing prism 50 are respectively formed by injection molding. [0051] In the frame mounting rib 10k, the mounting portion lOg is arranged outside the eye cover unit EC, and has a planar part that abuts against an abutting surface disposed in the frame and is a reference determining a relative posture during the mounting onto the above-described and not-shown frame. [0052] As shown in Fig. IB, not only the lateral end portion SP where the rib 60 is disposed but also a tapered surface TP that is formed to surround the entire first surface SI 1 which is a part of the second face SS2 is disposed between the first face SSI and the second face SS2. The tapered surface TP is configured to have an inclined surface that forms an obtuse angle with respect to the first surface SI 1, and has an inclined side surface that extends along a part of the face of the wearer in wearing. In this manner, the light guide device 20 can have an optical design to be closer to the eye of the wearer. More specifically, the tapered surface TP has three flat surface parts S31, S32, and S33 as the inclined side surface, each of which corresponds to an eye socket edge, a cheek, and a nose. [0053] As shown in Fig. 1A, the light guide device 20 has a connection surface CS1 between the first face SSI and the flat plate portion FBI, and a connection surface CS2 between the first face SSI and the flat plate portion FBI. The connection surfaces CS1 and CS2 are surfaces respectively inclined with respect to surfaces vertical to the flat plate portions FBI and FB2. For example, by having the connection surfaces CS1 and CS2 inclined a few degrees with respect to the surfaces vertical to the flat plate portions FB 1 and FB2, a cut taper can be provided during the molding and transfer accuracy of the first and second faces SSI and SS2 can be increased. [0054] As described above, the light guide device 20 is formed to have a number of freeform surfaces and, particularly, the second surface S12 of the light guide prism 10 that is the bonding site, that is, the first bonding surface AS, and the second transmitting surface S52 of the opposing prism 50 that has substantially the same shape corre sponding thereto, that is, the second bonding surface BS are also free-form surfaces. Accordingly, in manufacturing of the light guide device 20, it is important to ensure a state where accuracy is high at the bonding site even for the bonding of such free-form surfaces and appropriately control the adhesive to be the joint JN during the assembly without causing bubbles to remain for the filling in a desired state. In this embodiment, particularly in a process of bonding the light guide prism 10 with the opposing prism 50 in manufacturing the light guide device 20, workability of the bonding is increased and it is allowed for each of the members to ensure the optical function. [0055] Fig. 2(A) is a side sectional view of the light guide device 20 in a state where the light guide prism 10 and the opposing prism 50 are bonded with each other. (B) and (C) in Fig. 2 are schematic views illustrating an overview with regard to a state of a clearance of the bonding site during a bonding process in manufacturing the light guide device 20. In this embodiment, as shown in (B) and (C) in Fig. 2, the light guide prism 10 and the opposing prism 50 approach each other from a direction shown with an arrow Al to be adhered and bonded with each other during the manufacturing of the light guide device 20. Hereinafter, the direction shown with the arrow Al is the specific offset direction DD. Herein, a posture (inclination) of the first bonding surface AS and the posture (inclination) of the second bonding surface BS are adjusted to be in a substantially exactly overlapping state in a case where the sliding movement in the offset direction DD is continued. In the case of the adhesion through the approach from the offset direction DD, a difference can be made with regard to a distance between the first bonding surface AS and the second bonding surface BS, that is, the clearance between the two, as shown in an area Ql on a lower side and an area Q2 on an upper side of (B) and (C) in Fig. 2. As shown in the drawings, when the first bonding surface AS that is positioned relatively on a left side is a convex surface looking upward to the right, the second bonding surface BS that is positioned relatively on a right side is a concave surface looking upward to the right, the first bonding surface AS is the upper side, and the second bonding surface BS is the lower side, with regard to the clearance, a distance Tl that shows a size of the clearance in the area Ql is shorter than a distance T2 in the area Q2 as shown in Fig. 2(B). Accordingly, during a process of causing the light guide prism 10 and the opposing prism 50 to approach each other in the offset direction DD shown with the arrow Al which is a lateral direction (left-right direction) to put these into a state shown in Fig. 2(C), the clearance on the area Ql side p o sitioned on the lower side is narrower in an early stage than on the area Q2 side p o sitioned on the upper side. The first bonding surface AS and the second bonding surface BS have substantially the same shape and, when these approach each other, these finally have the postures substantially overlapping with each other, and thus the clearance difference can be substantially removed if an area of the adhesion area, that is, the joint is zero. However, during the process of the above-described approach, ways of narrowing the clearance are different depending on positions for relationship with the offset direction DD and a difference in the sizes of the clearances is made. According to a method for manufacturing the light guide device of this embodiment, the difference in the ways of narrowing the clearance is used such that an adhesive CC that is applied to a central side on the lower end side on the second bonding surface BS can be controlled to flow mainly in a direction Dl directed from the lower end side to the upper end side shown with an arrow within a surface of the second bonding surface BS (or the first bonding surface AS) and also in a direction D2 and a direction D3 as shown in, for example, Fig. 2(D). In the above-described manner, during the bonding between the light guide prism 10 and the opposing prism 50, the adhesive CC can be completely filled between the first bonding surface AS and the second bonding surface BS such that the bonding can be made with high accuracy. In the case shown in Fig. 2(A), a width HI of an end portion of the joint JN on a side close to the eye of the observer is larger than a width H2 of an end portion on a far side. This is because the side close to the eye is likely to be blurred for the observer and is comparatively unlikely to be noticed even when a width of the joint JN is relatively increased. In this case, during the bonding process, that is, the process of forming the joint JN, the joint JN that is formed can be unlikely to pose inconvenience to a field of vision even when the amount of the adhesive to be the joint JN is increased sufficiently to spread throughout an adhesion range. In addition, a positioning part TT is formed on an end portion side of the joint JN that is close to the eye. The positioning part TT is a part that is formed by slightly bending upper end side end portions of the first bonding surface AS and the second bonding surface BS, and prevents a misreg istration from occurring by pressing the light guide prism 10 and the opposing prism 50 to each other during the bonding. In a case where there is no concern over the mis registration during the bonding, the positioning part TT, that is, the bent part of the first bonding surface AS and the like may not be disposed. [0057] During the bonding between the light guide prism 10 and the opposing prism 50, a curvature of the second bonding surface BS with respect to a curvature of the first bonding surface AS is adjusted if necessary such that an adjustment of the difference in the sizes of the clearances between the light guide prism 10 and the opposing prism 50 is considered. Specifically, first, a shape of the first bonding surface AS that is the freeform surface and a surface equivalent to the second surface S12 which is an optical function surface for the light guiding is expressed by the following formula (1). Herein, x, y, and z are coordinate values in polar coordinates, c is the curvature, A is each coefficient parameter, and r0 is a normalized aperture. With respect thereto, a flow direction of the adhesive CC can be controlled by adjusting, for example, the c and the A that is the parameter in the above formula as necessary and defining the shape of the second bonding surface BS. By performing the adjustment, controlling to accelerate the flow can be further reliably performed in not only the direction Dl corresponding to the offset direction DD but also the directions of the direction D2 and the direction D3 with respect to the flow direction of the adhesive CC shown in, for example, Fig. 2(D). [0058] With regard to a positional relationship between the first bonding surface AS and the second bonding surface BS, the first bonding surface AS is the upper side of the drawing and the second bonding surface BS is the lower side as described above and later description will be made on the assumption of this relationship but, as in the case of, for example, a modification example shown in Fig. 2(E), the first bonding surface AS can be the lower side and the second bonding surface BS can be the upper side with both thereof being arranged to look upward to the left. In addition, in this case, the adhesive CC may be applied to the upper end side of the first bonding surface AS. [0059] In addition, with regard to a method for manufacturing the joint JN, that is, a way of making the clearance, another aspect is also considered. As an example, as with a com parative example shown in Fig. 2(F), not using the clearance difference caused by the adhesion from the offset direction is also considered by making the difference in the clearances depending on the positions by only changing the curvature of the second bonding surface BS with respect to the curvature of the first bonding surface AS during the bonding between the light guide prism 10 and the opposing prism 50. Specifically, in the formula (1) above, it is also considered that a shape of the free-form surface that is re-calculated by slightly decreasing, for example, a value of the curvature shown with the c in the formula is expressed as the shape of the curved surface of the second bonding surface BS which is a surface corresponding to the first bonding surface AS such that the curvature of the second bonding surface BS with respect to the curvature of the first bonding surface AS is changed and a desired clearance is formed. In other words, as shown in Fig. 2(F), it is considered that the clearance is increased away from a minimum clearance position MP with the minimum clearance position MP shown with a dashed line being a reference, that is, the distance T2 and the distance T3 that are distances of the clearance at positions apart from the minimum clearance position MP are increased with respect to the distance Tl of the clearance at the minimum clearance position MP. However, in a case where such design is performed, arithmetic processing to define the free-form surface as described above is necessary for the first bonding surface AS and the second bonding surface BS so as to obtain a shape with which the adhesive CC is controlled into a desired state, that is, there is a case where the shape of the free-form surface needs to be designed for substantially each of the surfaces. With respect thereto, in this embodiment, the first bonding surface AS and the second bonding surface BS have substantially the same shape in principle, and the shapes can be defined with regard to both of the first bonding surface AS and the second bonding surface BS if the calculation to determine the free-form surface is performed on the second surface S12 which is the optical function surface, and thus the light guide prism 10 and the opposing prism 50 can be manufactured in a simpler manner compared to the case shown in Fig. 2(F). In addition, even in the bonding, the flow is expanded from the minimum clearance position MP in the case shown in Fig. 2(F), but there is a possibility of further complexity as the flow direction includes not only the main direction Dl directed, for example, from the lower end side to the upper end side but also, reversely, a direction directed from the lower end side to the upper end side and the like. With respect thereto, in this embodiment, the flow of the adhesive CC is simplified and the filling can be more reliably performed. [0060] Hereinafter, the method for manufacturing the light guide device 20 according to this embodiment will be described in detail referring to Figs. 3 and 4. As described above, in this embodiment, a process of preparing the light guide prism 10 and the opposing prism 50 during the manufacturing process and a process until completing the bonding therebetween are particularly important. Accordingly, hereinafter, a process from the molding of each of the prisms 10 and 50 to the forming of the joint will be described. [0061] First, referring to Fig. 3, the molding (molding process) of each of the prisms by injection molding using a molding die in the process of manufacturing the light guide device 20 will be described. Herein, as an example, the molding of the light guide prism 10 that constitutes the light guide device 20 will be described. The description is similar with regard to the opposing prism 50, and thus illustration and description will be omitted. [0062] As shown in Fig. 3, a molding die 90 that is used to mold the light guide prism 10 is configured to have a first die 9 1 and a second die 92, and the light guide prism 10 can be molded by sandwiching the first die 9 1 and the second die 92 and clamping both of the dies 9 1 and 92. In other words, molten resin flows from a gate GT that is a resin inlet into a cavity CV with respect to the molding die 90 in which the cavity CV which is an inner space in the clamped state is formed, and a suitable operation such as heating, cooling, pressing, and depressurization is performed such that the light guide prism 10 which has each of the curved surfaces such as the above-described free-form surface is formed (molding process). Herein, as shown in the drawing, the molding die 90 has transfer surfaces tailored to the surfaces. Specifically, in the first and second dies 9 1 and 92, the transfer surfaces S91 that are free-form surfaces which are formed to oppose each other correspond to the first surface SI 1, third surface S13 (refer to Fig. 1A and the like), and the like contributing to the light guiding. In addition, the transfer surfaces S92 correspond to the tapered surface TP that has the flat surface parts S31, S32, S33, and the like. Also, the transfer surfaces S93 correspond to the connection surfaces CS1 and CS2, and the transfer surfaces S94 and S95 correspond to the flat plate portions FBI and FB2, that is, the faces of the bonding ribs 10h and lOp. In other words, the rib 60 that has the first bonding rib Rl and the like is also integrally formed. What remain after cutting in a part of the gate GT are gate portions GP1 and GP2. [0063] Herein, as shown in the drawing, of the transfer surfaces S94 and S95 that correspond to the flat plate portions FBI and FB2 (first and second flat plate portions), the transfer surface 95 is along a parting line PL of the molding die 90, and the transfer surface S94 is along the parting line PL in a parallel manner. In other words, the flat plate portions FBI and FB2 are disposed along the parting line PL. In this case, accuracy of planar parts of the faces of the flat plate portions FB1 and FB2 can have the same degree of accuracy as the optical function surface. Accordingly, the bonding ribs 10h and lOp of the rib 60 can have simplicity and high accuracy, and can be used as reference surfaces during the bonding between the light guide prism 10 and the opposing prism 50. [0064] Hereinafter, the bonding between the light guide prism 10 and the opposing prism 50 will be described in detail referring to Fig. 4 and the like. Herein, as an example, the bonding is performed by using a mounting device 500. Fig. 5 is a cross-sectional view taken along line A-A of Fig. 4. [0065] First, a structure of the mounting device 500 that is used to bond the light guide prism 10 and the opposing prism 50 with each other will be described. As shown in Figs. 4 and 5, the mounting device 500 has a first moving mechanism 501a that is used to move the light guide prism 10 in a fixed posture, a second moving mechanism 501b that is used to move the opposing prism 50 in a fixed posture, a pair of communication shaft portions 502, a position sensor 503 that confirms the relative positional relationship between the light guide prism 10 and the opposing prism 50, and a control unit 510 that performs controlling on each of the portions. [0066] Herein, an extension direction of the pair of communication shaft portions 502 cor responds to the offset direction DD, that is, the pair of communication shaft portions 502 allow the sliding movement along the offset direction DD. Also, herein, con sidering that the adhesive CC is applied onto the second bonding surface BS, it is preferable that flow of the adhesive CC into an unintended direction due to gravity be suppressed to the maximum extent possible. Accordingly, as shown in the drawing, the first bonding surface AS (or the second bonding surface BS) is in a state close to horizontal as much as possible with respect to an exemplified gravity direction G or a part to which the adhesive CC is applied is on a slightly upper side with regard to the gravity direction G than the other parts. [0067] The first moving mechanism 501a has a main body portion 511a, fixing tools 512a, and a drive device 513a. The main body portion 511a fixes the posture of the light guide prism 10 in conjunction with the fixing tools 512a, and is inserted by the com munication shaft portions 502 such that the sliding movement is allowed. As shown in Fig. 5, the fixing tools 512a are a pair of members that are disposed to correspond to a pair of alignment groove portions DT which are disposed in the main body portion 511a. The first moving mechanism 501a places the flat plate portions FB 1 and FB2, that is, the bonding ribs 10h and lOp of the light guide prism 10 on the groove portions DT of the main body portion 511a, places the fixing tools 512a from thereabove, and fixes the fixing tools 512a with respect to the main body portion 511a through, for example, screwing and pinning or adsorption (not shown) by suction or the like such that each of the surfaces of the flat plate portions FB1 and FB2 of the bonding ribs 10h and lOp are aligned in a state of being pressed against and pinched by the groove portions DT. Specifically, the alignment is made in a direction from the first surface SI 1 to the third surface S13 of the light guide prism 10, that is, a direction EE shown with an arrow in the drawing by a planar part DTI of the groove portion DT and the fixing tool 512a, and the alignment is made in a direction from the flat plate portion FBI to the flat plate portion FB2 of the light guide prism 10, that is, a direction FF shown with an arrow in the drawing by a planar part DT2. In other words, the light guide prism 10 is fixed in a state of being aligned to the main body portion 511a with a desired posture. The drive device 513a is configured to have a motor or the like, and causes the main body portion 5 1l a to be slidingly moved along the communication shaft portions 502 in the arrow Al direction according to a command from the control unit 510, that is, causes the light guide prism 10 to be slidingly moved along the offset direction DD. [0068] The second moving mechanism 501b has a similar function as the first moving mechanism 501a, and has a main body portion 5 1lb, fixing tools 512b, and a drive device 513b. in other words, as is the case with the first moving mechanism 501a, the second moving mechanism 501b fixes the posture of the opposing prism 50 to the main body portion 5 1lb in a desired state and moves the main body portion 5 1lb such that the opposing prism 50 whose posture is maintained is slidingly moved in the offset direction DD. [0069] The communication shaft portions 502 are members that are configured to have a pair of shaft portions which extend in parallel with each other, and respectively cause the main body portion 5 1l a of the first moving mechanism 501a and the main body portion 5 1lb of the second moving mechanism 501b to communicate with each other, and allow the sliding movement in parallel extension directions. Herein, the extension direction of the pair of the communication shaft portions 502 corresponds to the offset direction DD as described above, that is, by the communication shaft portions 502, the light guide prism 10 and the opposing prism 50 can be slidingly moved along the offset direction DD in a state where the faces of the flat plate portion FB 1 and the flat plate portion FB2 are arranged on the same plane. [0070] The position sensor 503 is configured to have an optical sensor that detects positions of the light guide prism 10 and the opposing prism 50 by using, for example, reflection of laser light and the like, a pressure sensor that allows pressure which is applied between the light guide prism 10 and the opposing prism 50 to be measured, or the like, and is a sensor that allows the control unit 510 to be informed of an adhesion condition between the light guide prism 10 and the opposing prism 50 by confirming the relative positions of the light guide prism 10 and the opposing prism 50. [0071] In the above description, the bonding ribs 10h and lOp are references for the fixing of the light guide prism 10 by the first moving mechanism 501a and the fixing of the opposing prism 50 by the second moving mechanism 501b. In this manner, the relative positions of the light guide prism 10 and the opposing prism 50 are precise. [0072] Further, in the case of the above-described structure, the direction of the sliding movement is the offset direction DD. In this manner, during the adhesion process, the flow of the adhesive is controlled, and the filling of the adhesive can be appropriately performed. [0073] Hereinafter, each of the processes in the manufacturing of the light guide device 20 by using the mounting device 500 will be described. First, the light guide prism 10 and the opposing prism 50 that are molded (molding process) by the molding die 90 or the like which has been described by using Fig. 3 are mounted on the first moving mechanism 501a and the second moving mechanism 501b of the mounting device 500 and are fixed (preparation process) in a state of being capable of approaching in the offset direction DD and in a state where the first bonding surface AS and the second bonding surface BS oppose each other in a desired posture (preparation process). Then, the adhesive CC is applied (adhesive application process) onto the second bonding surface BS. Herein, an appropriate amount of the adhesive CC is applied to the lower end side of the second bonding surface BS. Then, the drive device 513a of the first moving mechanism 501a and the drive device 513b of the second moving mechanism 501b are operated via the control unit 510 to cause the light guide prism 10 and the opposing prism 50 to move in the offset direction DD and approach each other, and both are adhered with each other (adhesion process) while the relative position is confirmed by the position sensor 503. In this case, the adhesive CC is appropriately controlled such that the adhesion area is completely filled therewith. Lastly, the adhesive CC is cured to form the joint JN, the light guide prism 10 and the opposing prism 50 are connected with each other (bonding process), and the light guide device 20 is manufactured. Various methods can be considered with regard to the curing of the adhesive CC. For example, in a case where the adhesive CC is ultraviolet curable resin, the adhesive CC is cured through ultraviolet irradiation in a state where the posture during the adhesion process is maintained by the mounting device 500. In the above description, particularly during the adhesion process, an inclination angle of the first bonding surface AS with respect to the offset direction DD is adjusted, and thus the filling of the adhesive CC can be performed in a desired state even with a relatively simple operation such as the sliding movement (linear movement) in the offset direction DD. Fig. 6 is a view showing the inclination angle of the first bonding surface AS with respect to the offset direction DD. In this embodiment, the first bonding surface AS that is the bonding site of the light guide prism 10 has a convex surface shape, and the first bonding surface AS is the free-form surface, and thus the inclination angle with respect to the offset direction DD is also different at each position on the first bonding surface AS. Herein, the inclination angle of the offset direction DD with respect to the first bonding surface AS means an angle formed by a directional vector showing the offset direction DD with respect to a plane defining the direction of the first bonding surface AS at each point of the first bonding surface AS, which is, for example, a tangent plane at the point. Particularly, in the state that is shown in the drawing, the first bonding surface AS is convex with regard to a downward direction of the drawing whereas the offset direction is the lateral direction of the drawing. As such, with regard to the inclination angle, an inclination angle alpha of the first bonding surface AS on a downward direction side (one end side) of the drawing is relatively smaller, and an in clination angle gamma on an upward direction side (the other end side) of the drawing is relatively larger. In addition, an inclination angle beta on a middle side of the first bonding surface AS is larger than the inclination angle alpha and is smaller than the inclination angle gamma. In other words, with regard to the angle, alpha