KEYBOARD APPARATUS OF ELECTRONIC MUSICAL INSTRUMENT BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a keyboard apparatus of an electronic musical instrument, and more particularly to a stopper member which restricts pivoting of keys of the keyboard apparatus and massive bodies which pivot in synchronization with the keys. Description of the Related Art In a keyboard apparatus of an electronic musical instruments, conventionally, a range in which keys are allowed to pivot when the keys are depressed (from the initial position to the maximum pivotable position) is restricted by an upper limit stopper and a lower limit stopper provided on a key frame. FIGS. 20A to 20C are right side views schematically illustrating a conventional keyboard apparatus of an electronic musical instrument. The keyboard apparatus has a white key main body 1, a black key main body 2 and a key frame 3. The key frame 3 has front and rear steps in a longitudinal direction of the keys to have a horizontal portion 3a between the steps. On the rear end of the horizontal portion 3a, a key supporting portion 3b is provided. On the rear end of the white key main body 1, a key pivot portion 1 b Is provided. The white key main body 11s fastened to the key supporting portion 3b with a key pivot portion 1b to pivot about the key pivot portion 1b. The white key main body 1 has right and left sides formed downward from the both sides of the top surface. The white key main body 1 also has a lA pair of right and left stopper pieces 1c provided at the midpoint near a front end 1a of the right and left sides, the stopper pieces hanging downward from the right and left sides. Front ends 1 d of the stopper pieces 1 c are bent approximately perpendicularly toward the depth of the key. The black key main body 2 also has a key pivot portion 2b and stopper pieces 2c on the positions similar to those of the white key. On the front step of the key frame 3, a plurality of parallel slits 3c are vertically formed so that the front ends 1d, 2d of the stopper pieces of the white key and the black key are inserted into the respective slits 3c. On the horizontal portion 3a of the key frame 3, a key switch 4 is provided. Projections (actuators) which are not shown are provided on the undersurface of the white key main body 1 and the black key main body 2 to oppose to the key switch 4. On the undersurface of the horizontal portion 3a of the key frame, an upper limit stopper 182 which is shaped like a stripe is arranged along the keys so that the upper limit stopper 182 opposes to the top surface of the front ends 1 d of the stopper pieces 1 c. On the top surface of the horizontal portion 3a, a lower limit stopper 181 is arranged so that the lower limit stopper 181 Is shaped like a stripe. In the shown example, the white keys and the black keys have the same structure. In view of shock absorbency, noise reduction and reproducibility of pivotable range, the upper limit stopper 182 and the lower limit stopper 181 are required to be formed of material having elastic recovery force. Conventionally, felt, polyurethane elastomer or the like is employed as the material of the upper limit stopper and the lower limit stopper. When a player depresses a key, the key switch 4 is eiastically compressed to turn on a switch. When the key is further depressed, the lower limit stopper 181 is 2 given a shock by the right and left sides of the white key main body 1 or the black key main body 2 to be elastically deformed. At the time of elastic deformation, internal friction causes kinetic energy to be converted into heat energy, resulting in damping of the key. When the lower limit stopper 181 recovers from elastic deformation, the lower limit stopper 181 produces elastic recovery force. This elastic recovery force becomes repulsion (referred to as rebound) for the key. The rebound is conveyed to the player's finger through the white key main body 1 or the black key main body 2, causing player discomfort. When the key is released, a force which recovers the key such as action of a recovery spring which Is not shown causes the white key main body 1 or the black key main body 2 to return to its initial position. When the key returns, the top surface of the front ends 1d of the stopper pieces 1c collide with the upper limit stopper 6. At this collision, an elastic recovery force of the upper limit stopper 6 becomes repulsion for the white key main body 1 or the black key main body 2. Since this repulsion Is exerted toward the direction opposite to the force which recovers the key, the white key main body 1 or the black key main body 2 vibrates until the key fully stops. If the player's finger touches the key, as a result, the vibration is conveyed to the player to cause discomfort. In addition, there has been a conventional keyboard apparatus of an electronic musical instrument whose massive bodies pivot in synchronization with player's key-depressions to provide the player with key touch close to that of an acoustic piano. The upper limit of a pivoting massive body is restricted by a collision of the vicinity of a rear free end of an inertial moment generating portion with an upper limit stopper. When a 3 key is released to return to its initial state, the position of the massive body is restricted by a collision of the vicinity of the rear end of the inertial moment generating portion with a lower limit stopper. The upper limit stopper and the lower limit stopper provided for the above-described massive body also absorb shock of inertial moment of the massive body to damp the massive body. The upper limit stopper produces a repulsion when the massive body is driven to collide with the upper limit stopper, while the lower limit stopper produces a repulsion when the massive body recovers to collide with the lower limit stopper. In an invention disclosed in Japanese Patent Laid-open No. 2003-195853, for an upper limit stopper provided for hammers (massive bodies), a "massive portion MB" is provided along with the upper limit stopper. In the "massive portion MB", a massive body is sandwiched between a lower cushion and an upper cushion. This massive body is made of elastic material made by blending elastomer or rubber with metal powder so that the material has an appropriate mass and flexibility. In addition, Japanese Patent Laid-Open No. 2003-195853 also discloses that the "massive portion MB" may employ a chain metal wire or a member formed by packing sand or metal powder in a pouched member so that the member has the same width as the entire width of the keys or the width of a plurality of keys. The "massive portion MB" is fastened to a rear part of a chassis by bonding or the like, or is held by enfolding the "massive portion MB" from below upward with a sheet member. DISCLOSURE OF THE INVENTION Problems to Be Solved by the Invention 4 In the invention disclosed in Japanese Patent Laid-Open No. 2003-195853, the massive body MB receives an impact from a hammer through the lower cushion. The massive body MB is bonded to be fastened to the chassis through the upper cushion. Therefore, the member formed by packing sand or metal powder in the pouched member does not fully exert its ability. In Japanese Patent Laid-Open No. 2003-195853, in addition, the concrete structure for packing sand or metal powder in the pouched member is not studied. For instance, the structure of the packing member which packs sand or metal powder and the concrete structure applied to a case where conventional stopper members using felt are employed along with the massive portions MB are not studied. In addition, the long-term stability of sand or metal powder packed in the packing member is not studied. Furthermore, cases where the sand or metal powder packed in the packing member travels to unexpected areas due to resting of the musical instrument against a wail or carriage of the musical instrument are not studied. The present invention was accomplished to solve the above-described problems, and an object thereof is to provide a keyboard apparatus of an electronic musical instrument having an action restricting member which restricts a range in which pivoting members such as keys and massive bodies are allowed to pivot, the action restricting member having a concrete structure which improves key touch to be perceived by a player. In addition, another object of the present invention is to provide a keyboard apparatus of an electronic musical instrument having an action restricting member which restricts a range in which pivoting members such as keys and massive bodies are allowed to pivot, the action restricting 5 member having a concrete structure which enables the action restricting member to retain its ability over the long term in a case where an enclosure member which encloses a plurality of grains is employed as the action restricting member. Means for Solving Problems and Technical Effect It is a feature of the present invention to provide a keyboard apparatus of an electronic musical instrument comprising a plurality of pivoting members each of which pivots in response to a depression or release of a key; a frame which supports the pivoting members such that the pivoting members are capable of pivoting; and an action restricting member which causes collision thereof with the pivoting members to restrict a range in which the pivoting members are allowed to pivot, wherein the action restricting member contains a plurality of grains in its closed area partially or entirely enclosed with an enclosure member, and is arranged, in a state where inflow and outflow of air are allowed between the closed area and outside air, at the frame's side along a direction in which the pivoting members are arranged. In this case, the enclosure member has breathability, for instance. Furthermore, the enclosure member is fastened to a breathable base so that the enclosure member and the breathable base form the enclosed area. In the keyboard apparatus configured as above, when a pivoting member collides with the action restricting member, the grains slide and collide with each other, resulting in significant internal losses. Since inflow and outflow of air are allowed between the closed area and outside air, in addition, any repulsion force that would be caused by the internal air pressure of the enclosure member will not be produced 6 when the pivoting member collides with the action restricting member. As a result, any repulsion force will not be exerted on the pivoting member by the action restricting member. Resultantly, the keyboard apparatus of this feature improves key touch to be perceived by a player when he operates the keyboard apparatus. The enclosure member is a mesh enclosure member, for example. It is preferable that the mesh enclosure member is designed such that dimensions of a mesh opening are smaller than those of the minimum cross-sectional shape of the grains so that the mesh enclosure member can prevent the grains from passing through the mesh openings. The enclosure member may be a thin film having a plurality of air vents, for example. It is preferable that the thin film is designed such that dimensions of an opening of the respective air vents are smaller than those of the minimum cross-sectional shape of the grains so that the thin film can prevent the grains from passing through the air vents. The enclosure member is designed, for example, such that the surface of the enclosure member expands and contracts in accordance with a force externally exerted on the enclosure member. In this case, the action restricting member is able to contain a plurality of grains in its enclosed area without hindering sliding and collisions of the grains enclosed with the enclosure member. In a case where the base is breathable, the thin film employed as the enclosure member may not have the air vents. In some cases, the enclosed area is entirely enclosed with the enclosure member. In the other cases, the enclosed area is entirely enclosed with the base and the enclosure member. In some cases, furthermore, the base is provided independently of the frame. In the other 7 cases, the base is the frame itself. In the keyboard apparatus as configured above, points which are located on the action restricting member and with which the pivoting members collide are situated below a position where the action restricting member is fastened to the frame's side. Because the grains gather downward due to gravity, the keyboard apparatus configured as above precisely maintains the points which are located on the enclosure member and with which the pivoting members collide, also precisely restricting the initial position, the maximum pivotable position and the pivotable range of the pivoting members. In the keyboard apparatus configured as above, the keyboard apparatus further comprises an elastic action restricting member for exerting a repulsion force larger than that of the action regulating member; wherein the action restricting member and the elastic action restricting member are situated at the frame's side such that when the pivoting members pivot, the pivoting members collide with the elastic action restricting member before colliding with the action restricting member. The elastic action restricting member which produces repulsion force has larger recovery force for recovering to its original form. Therefore, it is preferable that the keyboard apparatus Is designed such that the pivoting members collide with the elastic action restricting member before colliding with the action restricting member in order to ensure precise positions where the pivoting members collide. As a result, the keyboard apparatus configured as above ensures precise restriction on the initial position, the maximum pivotable position and the pivotable range of the pivoting members. When the pivoting members collide with the action restricting member, the action restricting member 8 absorbs collision energy without producing repulsion force. In addition, some keyboard apparatuses have massive bodies which pivot in synchronization with depression of keys, however, the other keyboard apparatuses do not have massive bodies. The action restricting member in which the grains are enclosed with the breathable enclosure member may be employed for restriction on the pivotable range of either keys or the massive bodies. Alternatively, the action restricting member may be employed for restriction on the pivotable range of both the keys and the massive bodies. Since the Inertial moment of the massive bodies is larger than that of the keys, the action restricting member is more effective in a case where the action restricting member is employed for restriction on the pivotable range of the massive bodies. The action restricting member which does not employ action restricting member in which the grains are enclosed with the breathable enclosure member may employ a conventional elastic action restricting member. The restriction on the pivotable range includes restriction on the upper limit position and that on the lower limit position. The action restricting member in which the grains are contained in its closed area with the breathable enclosure member may be employed for the restriction either on the upper limit position or on the lower limit position. Alternatively, the action restricting member may be employed for the restriction on both positions. In a case where the action restricting member is employed for the restriction on the pivoting caused by key depressions (generally, the restriction on the lower limit position for keys and the restriction on the upper limit position for massive bodies), particularly, the keyboard apparatus configured as above significantly improves key touch to be perceived by a player, for the action restricting 9 member works in a state where the player depresses keys with hfs fingers. It is another feature of the present invention to provide a keyboard apparatus of an electronic musical instrument comprising a plurality of pivoting members each of which pivots in response to a depression or release of a key; a frame which supports the pivoting members such that the pivoting members are capable of pivoting; and an action restricting member which causes collision thereof with the pivoting members to restrict a range In which the pivoting members are allowed to pivot; wherein the action restricting member contains a plurality of grains in its closed area partially or entirely enclosed with an enclosure member, and is arranged at the frame's side along a direction in which the pivoting members are arranged; and the closed area enclosed with the enclosure member is provided with a travel restricting member for restricting travel of the grains. In the keyboard apparatus configured as above, when a pivoting member collides with the action restricting member, the grains slide and collide with each other, resulting in significant internal losses. As a result, a repulsion force exerted on the pivoting member by the action restricting member is small. Resultantly, the keyboard apparatus of this feature improves key touch to be perceived by the player when he operates the keyboard apparatus. Because the travel restricting member restricts travel of the grains in the direction in which the pivoting members are arranged, in addition, the keyboard apparatus ensures the ability of the action restricting member over the long term. In the keyboard apparatus configured as above, the travel restricting member Is arranged such that a travel restricting portion is situated at least part of a plurality of locations which are midpoints between points which are 10 located on the enclosure member and with which the pivoting members collide. Because the pivoting members will not interfere with the travel restricting member, as a result, the clamping ability of the action restricting member is ensured. In the keyboard apparatus configured as above, the travel restricting member is provided such that each of a plurality of travel restricting portions is situated at respective locations which are midpoints between points which are located on the enclosure member and with which the pivoting members collide; and the travel restricting member has inclined walls whose lowest ends are points with which the pivoting members collide, and whose highest ends are locations where the travel restricting portions are located. According to this keyboard apparatus, when the pivoting members are no longer in a state of collision, gravity causes the grains to move toward the points with which the pivoting members collide, resulting in the grains recovering to their original state. in the keyboard apparatus configured as above, the travel restricting member is approximately vertically provided wall members. This keyboard apparatus has the simplest configuration for restricting travel of the grains in the direction in which the pivoting members are arranged. It is preferable that the action restricting member in which the wall members are located within the enclosure member is employed as an upper limit stopper, in particular, for the wall members will not hamper travel of the grains when the grains move upward due to collision of the pivoting members with the enclosure member and when the grains are drawn downward due to gravity. As a result, the keyboard apparatus ensures response of the action restricting member. 11 In the keyboard apparatus configured as above, the travel restricting member and the enclosure member are partially joined. This keyboard apparatus prevents the travel restricting member from moving within the enclosure member. In addition, the keyboard apparatus also avoid moving of the enclosure member itself and deformation of cross-sectional shape of the closed area. In the keyboard apparatus configured as above as well, the pivoting members are at least either keys or massive bodies which pivot in synchronization with depression of the keys. It is still another feature of the present invention to provide a keyboard apparatus of an electronic musical instrument comprising a plurality of pivoting members each of which pivots in response to a depression or release of a key; a frame which supports the pivoting members such that the pivoting members are capable of pivoting; and an action restricting member which causes collision thereof with the pivoting members to restrict a range in which the pivoting members are allowed to pivot, wherein the action restricting member contains a plurality of grains in its closed area partially or entirely enclosed with an enclosure member, and is arranged, in a state where a plurality of sealing portions are formed on the enclosure member along a direction in which the pivoting members are arranged, at the frame's side along the direction in which the pivoting members are arranged. In the keyboard apparatus configured as above, when a pivoting member collides with the action restricting member, the grains slide and collide with each other, resulting in significant internal losses. As a result, a repulsion force exerted on the pivoting member by the action restricting member is small. Resultantly, the keyboard 12 apparatus of this feature improves key touch to be perceived by the player when he operates the keyboard apparatus. Because the closed area of the action restricting member is divided into a plurality of areas by the sealing portions, the keyboard apparatus prevents the grains from traveling in the direction in which the pivoting members are arranged. As a result, the keyboard apparatus ensures the ability of the action restricting member over the long term. Even in a case where the keyboard apparatus leans against a wall or in a case where the keyboard apparatus is held longitudinally for carriage, furthermore, the keyboard apparatus ensures the ability of the action restricting member. In a case where the sealing portions are formed by members for squeezing the enclosure member from outside of the enclosure member, the keyboard apparatus reliably maintains the sealing portions over the long term. In a case where the sealing portions are formed by physical processing of the enclosure member itself, furthermore, the keyboard apparatus reliably maintains the sealing portions over the long term. In the case of the sealing portions formed by physical processing of the enclosure member, in addition, the formation of the sealing portions is easy. In the keyboard apparatus configured as above, the enclosure member is formed to be shaped like a tube along a direction in which keys are arranged. According to this keyboard apparatus, the formation of the enclosure member is easy, while the handling of the grains is also easy. In the keyboard apparatus configured as above, the enclosure member Is formed by folding a rectangular flat material in two and then joining ends of the rectangular flat material to be shaped like a tube. Therefore, the enclosure member is allowed to be formed by an inexpensive 13 rectangular flat material. In the keyboard apparatus configured as above, the sealing portions are formed by partially squashing the enclosure member in a direction approximately orthogonal to the direction in which the pivoting members are arranged, and then superposing and joining two layers to approximately form a line; and the action restricting member is arranged at the frame's side such that the pivoting members collide with a plurality of points located on the enclosure member at an angle approximately orthogonal to the sealing portions. According to this keyboard apparatus, because the respective points which are located on the sealing portions and with which the pivoting members collide have the same deformed shape, distortion in shape will not occur. As a result, the keyboard apparatus ensures the abilities of the action restricting member. In the keyboard apparatus configured as above, the sealing portions are formed by squeezing the enclosure member such that the sealing portions are approximately shaped like a dot. According to this keyboard apparatus, even if the pivoting members collide with the enclosure member at any angle, distortion in shape caused by the sealing portions will not occur. As a result, the keyboard apparatus ensures the ability of the action restricting member. In the keyboard apparatus configured as above, the sealing portions are provided on at least part of a plurality of locations which are midpoints between points with which the pivoting members collide, the points being located on the enclosure member. Because the pivoting members will not interfere with the sealing portions, the ability of the action restricting member is ensured. 14 In the keyboard apparatus configured as above as well, the pivoting members are at least either keys or massive bodies which pivot in synchronization with depression of the keys. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A to 1C are explanatory drawings illustrating a first embodiment of the present invention; FIG. 2 is a right side view schematically illustrating a second embodiment of the present Invention; FIGS. 3A to 3C are explanatory drawings illustrating behavior of an enclosure member used in the embodiments shown in FIGS. 1A to 1C and FIG. 2; FIGS. 4A to 4F are explanatory drawings illustrating concrete examples of the enclosure member used in the embodiments shown in FIGS. 1A to 1C and FIG. 2; FIG. 5 is an explanatory drawing illustrating a third embodiment of the present invention; FIG. 6 is an explanatory drawing illustrating a fourth embodiment of the present invention; FIGS. 7A, 7B are explanatory drawings illustrating action of gravity exerted on grains provided for an upper limit stopper; FIGS. 8A to 8C are explanatory drawings illustrating a concrete example of a lower limit stopper shown in FIG. 2, the example showing an action restricting member having a travel restricting member inside; FIGS. 9A, 9B are explanatory drawings illustrating different concrete examples of the lower limit stopper shown in FIG. 2, the examples showing is an action restricting member having a travel restricting member inside; FIGS. 10A, 10B are explanatory drawings illustrating further different concrete examples of the lower limit stopper shown in FIG. 2, the examples showing an action restricting member having a travel restricting member inside; FIGS. 11A to 11D are explanatory drawings illustrating a concrete example of the upper limit stopper shown in FIG. 2, the example showing an action restricting member having a travel restricting member inside; FIGS. 12A to 120 are explanatory drawings illustrating different concrete examples of the upper limit stopper shown in FIG. 2, the examples showing an action restricting member having a travel restricting member inside; FIG. 13 Is an explanatory drawing Illustrating a further different concrete example of the upper limit stopper shown in FIG. 2, the example showing an action restricting member having a travel restricting member inside; FIG. 14 is an explanatory drawing illustrating a concrete example of the lower limit stopper shown in FIG. 2, the example showing an action restricting member having a travel blocking member outside; FIGS. 15A, 15B are explanatory drawings illustrating a concrete example of the lower limit stopper shown in FIG. 2, the example showing an action restricting member whose enclosure member has sealing portions; FIGS. 16Ato 16C are explanatory drawings illustrating different concrete examples having a tubular enclosure member shown in FIG. 15A, 15B; FIGS. 17A to 17E are explanatory drawings illustrating a keyboard 16 apparatus which employs tubular enclosure members having approximately linear sealing portions shown in FIGS. 15A, 15B; FIGS. 18 is an explanatory drawing illustrating a different concrete example which shows formation of sealing portions on a tubular enclosure member shown In FIGS. 15A, 15B; FIGS. 19A, 19B are schematic top views illustrating a concrete example of the lower limit stopper shown in FIG. 2, the top views showing an arrangement of the sealing portions formed on the enclosure member; and FIGS. 20A to 20C are right side views schematically illustrating a conventional keyboard apparatus of an electronic musical instrument. DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1A to 1C show a keyboard apparatus of an electronic musical instrument according to a first embodiment of the present invention. FIG. 1A is a right side view schematically illustrating the keyboard apparatus. FIG. 1B schematically illustrates behavior of grains provided for a lower limit stopper 5 viewed from the right side. FIG. 1C schematically illustrates behavior of grains provided for the lower limit stopper 5 viewed from the front. In FIGS. 1A to 1C, parts similar to those employed in FIG. 20 are given the same numbers as in FIG. 20 to omit detailed descriptions of the parts. The structure of the lower limit stopper 5 (action restricting member) and an upper limit stopper 6 (action restricting member) of FIG. 1A is different from those used in FIG. 20 described in the related art. The structure of the action restricting members of a black key main body 2 (pivoting member) of FIG. 1A is also different from that used in FIG. 20. 17 Referring to FIGS. 1B, 1C, a detailed description will be given with the lower limit stopper 5 provided for a white key main body 1 being taken as an example. The lower limit stopper 5 is partially or entirely enclosed with a breathable enclosure member 12 to contain a plurality of grains 11 in a closed area of the lower limit stopper 5. The lower limit stopper 5 is located on a horizontal part 3a of a key frame 3 (frame), the horizontal part 3a being used as a base of the lower limit stopper 5. The enclosure member 12 is sealed and fixed on the base. The lower limit stopper 5 extends in the direction in which a plurality of keys of the keyboard apparatus are arranged. The lower limit stopper 5 is not necessarily located on the key frame 3 itself, but may be located on anything which fixedly holds the keyboard apparatus as the key frame 3 does. The grains 11, which are solid objects, are shown as spherical objects. In a case where the grains 11 are spherical in shape, it is preferable that the grains 11 measure 3 mm or less in diameter. As described in Japanese Patent Laid-Open Publication No. 2003-19853, the grains 11 can be sand or metal powder. In addition, the grains 11 may also be spherical ceramic, spherical metal, spherical plastic or the like. However, the grains 11 are not necessarily spherical in shape. The enclosure member 12, which sweepingly restricts behavior of the grains 11 contained therein, is made of a thin flexible material (also referred to as membrane material). The degree of restriction can be freely determined in accordance with the degree of flexibility of the enclosure member 12, however, it is preferable to have such degree of restriction as will not prevent kinetic energy generated by a collision from propagating to the grains 11. More specifically, the enclosure member 12 is something 18 that allows the surface area of the enclosure member 12 to expand and contract In accordance with a force exerted on the enclosure member 12 due to the elasticity imparted to the material of the enclosure member 12. Alternatively, the enclosure member 12 may be something that flexibly deforms or loosens such as cloth. The enclosure member 12 is sealed and bonded or fusion-bonded to the horizontal portion 3a which is the base of the lower limit stopper 5, preventing the grains 11 from dispersing. When the lower ends of a left side portion 1e and a right side portion 1f of the white key main body 1 collide with the enclosure member 12, kinetic energy generated by the collision Is conveyed to the grains 11 through the enclosure member 12. When the grains 11 sequentially collide or slide with each other to move toward arbitral directions, the kinetic energy is converted to heat energy to disappear. Since elastic energy will not be accumulated, as a result, repulsion to a pivoting member will not be produced. Resultantly, the keyboard apparatus is able to eliminate uncomfortable key touch perceived by a player. Because the lower ends of the left side portion 1e and the right side portion 1f of the white key main body 1 directly collide with the grains 11 through the enclosure member 12, the effect of eliminating the kinetic energy is enhanced, the effect being produced by the moves of the grains 11. Since the breathable enclosure member 12 allows the grains 11 to be contained in a breathable area, in addition, elastic repulsion due to air pressure will not be produced. In the above description, the lower limit stopper 5 (action restricting member) is described, however, the upper limit stopper 6 (action restricting member) is also located on the underside of the horizontal part 3a of the key frame 3, the horizontal part 3a being used as the base of the upper limit 19 stopper 6. The enclosure member 12 of the upper limit stopper 6 is also sealed and fixed on the base. However, the relationship between the direction in which the grains 11 receives collision force and the direction in which the grains 11 receives gravity is different from that of the case of the lower limit stopper 5. Therefore, the upper limit stopper 6 basically has the same structure as the lower limit stopper 5, however, the structure can vary between the upper limit stopper 6 and the lower limit stopper 5. In this specification, the structure in which the grains 11 are enclosed (partially enclosed or entirely enclosed) with the enclosure member 12 will be simply referred to as "particle bag". At least either the lower limit stopper 5 or the upper limit stopper 6 may have the "particle bag" structure with the other stopper being composed of conventional elastic action restricting member (hereafter referred to as elastic action restricting member) made of felt or the like. FIG. 2 is a right side view schematically illustrating a keyboard apparatus of an electronic musical instrument according to a second embodiment of the present invention. The keyboard apparatus has a white key main body (pivoting member) 21, a black key main body (pivoting member) 22 and a key frame (frame) 23. On the key frame 23, step portions are provided in the forward part and the rear part in the longitudinal direction of the keys. Between the steps, a horizontal portion 23a is provided. On the rear part of the horizontal portion 23a, a key supporting portion 23b is provided. On a forward part of the underside of the horizontal portion 23a, a massive body supporting portion 23c is provided. On the rear end of the white key main body 21 and the black key main body 22, key pivot portions 21 b, 22b are provided. The white key main body 21 20 and the black key main body 22 are fastened to the key supporting portion 23b at the key pivot portions 21b, 22b, respectively, so that the white key main body 21 and the black key main body 22 can pivot about the key pivot portions 21b, 22b, respectively. In the front and the rear of the step portions of the key frame 23, mounting portions 23d, 23e are provided in order to mount the key frame 23 to a key frame bottom plate 24. The front of the mounting portion 23d is a vertical wall 23f. The key frame bottom plate 24 may be a fixing member of the key frame side such as a lower case (shelf board) of the electronic musical instrument. On the vertical wall 23f, a key guide 25 is provided for the white key main body 21. The key guide 25 is inserted into the lower portion of proximity of a front end portion 21a of the white key main body 21 in order to restrict lateral position and rolling of the key. A key guide 26 fixed to the horizontal portion 23a is provided for the black key main body 22. On the horizontal portion 23a of the key frame, the key switch 4 which is similar to that shown in FIG. 1 is provided. Projections (actuators) which are not shown are provided on the underside of the white key main body 21 and the black key main body 22 to oppose to the key switch 4. A force conveying portion 21 c protrudes from the lower part of the white key main body 21 to penetrate through a hole 23g provided on the horizontal portion 23a of the key frame. The force conveying portion 21 c has a bottom plate at its end. On the upper portion of the bottom plate, an aperture which opens to the rear side of the longitudinal direction of the key is provided. On the top surface and the undersurface of the bottom plate, elastic members 27 are fastened (the top surface is invisible). A massive body 28 is provided for each of the white key main bodies 21 and the black 21 key main bodies 22. More specifically, the massive bodies 28 are arranged beneath the respective keys in the direction in which the keys are arranged. The shown massive body 28, which is provided for the white key main body 21, is supported by a massive body supporting portion 23c so that the massive body 28 pivots through the force conveying portion 21c of the corresponding key. The massive body 28 has a pivot portion 28c supported by the corresponding massive body supporting portion 23c, a main driven portion 28a and a secondary driven portion 28b which are located forward of the pivot portion 28c and are engaged with the force conveying portion 21c of the key, and an armlike inertia! moment generating portion 28d which is located in the rear of the pivot portion 28c to generate inertial moment. The rear end of the inertial moment generating portion 28d is a mass concentrated portion 28e. The main driven portion 28a and the secondary driven portion 28b are engaged with the force conveying portion 21c such that the bottom piate of the force conveying portion 21c is sandwiched between the main driven portion 28a and the secondary driven portion 28b through the elastic member 27. When the massive body 28 pivots in synchronization with a player's depression of the key, a counteraction determined In accordance with inertial moment generated by the inertial moment generating portion 28d is imparted from the white key main body 21 to a player's finger. When the player releases his finger from the key, the massive body 28 pivots backward due to the action of gravity to return to the shown position. A force conveying portion of the black key main body 22, which is not shown, is located at a position where the force conveying portion Is 22 overlaid with the force conveying portion 21c in the direction of the depth of FIG. 2. For the black key main body 22 as well, therefore, a similar massive body is provided , being supported by a massive body supporting portion to allow pivoting of the massive body. The massive body for the black key main body 22 Is turned by the force conveying portion of the corresponding black key. On the top surface of the horizontal portion 23a, the lower limit stopper 5 for the keys is provided to be shaped like a strip as in the case of FIG. 1. An upper limit stopper (action restricting member) 30 for the massive body 28 is provided on the undersurface of the horizontal portion 23a of the key frame. The upper stopper 30 restricts the upper limit position of the massive body 28 by use of a collision with the top surface of the mass concentrated portion 28e when the massive body 28 pivots in synchronization with depression of the white key main body 21. A lower limit stopper (action restricting member) 29 for the massive body 28 is provided on the key frame bottom plate 24. The lower limit stopper 29 restricts the initial position of the massive body 28 by use of a collision with the undersurface of the mass concentrated portion 28e when the massive body 28 pivots in synchronization with release of the white key main body 21. The upper limit stopper 30 and the lower limit stopper 29 have the structure of "particle bag" as in the case of the upper limit stopper 6 and the lower limit stopper shown in FIG. 1. Since kinetic energy produced by collision of the massive body 28 is large compared to that produced by collision of a key, it is preferable that the upper limit stopper 30 and the lower limit stopper 29 are designed to have a larger "particle bag" so that 23 more energy is eliminated. The structure of the keyboard having massive bodies and the pivot direction are not limited to those shown in FIG. 2. In some keyboards, massive bodies inclined at an initial position are raised in the vertical direction when keys are depressed. Such keyboards may be designed such that the "particle bags" receive collision in the horizontal direction when the massive bodies are raised In the vertical direction. Alternatively, the massive bodies may be placed at the upper limit at the initial position with the massive bodies being placed at the lower limit when the keys are fully depressed. FIGS. 3A to 3C illustrate behavior of the enclosure member 12 used in the embodiments shown in FIGS. 1A to 1C and FIG. 2. An example of the lower limit stopper 29 shown in FIG. 2 will be described. The enclosure member 12 has such elasticity as allows the surface to expand and contract in accordance with internally and externally exerted force. FIG. 3A shows an initial state of such enclosure member 12. As shown in FIG. 3B, a collision force produced by a collision of the mass concentrated portion 28e with the lower limit stopper 29 causes the enclosure member 12 to expand in the horizontal direction, however, the enclosure member 12 exercises adequate contractive force to confine the grains 11 within a closed area and to entirely restrict behavior of the grains 11. By controlling flexibility of the enclosure member 12, the keyboard apparatus is able to offer player's desired key touch. As shown in FIG. 3C, when the mass concentrated portion 28e is released from the lower limit stopper 29, the expanding enclosure member 12 contracts due to its elastic recovery force to return to the initial state shown in FIG. 3A. Referring to FIGS. 4A to 4F, concrete examples of the enclosure 24 member and an example configuration of the base will be described. FIGS. 4A to 4F illustrate concrete examples of the enclosure member 12 used in the embodiments shown In FIGS. 1A to 1C and FIG. 2. In a concrete example shown in FIG. 4A, a mesh enclosure member 41 is employed as the enclosure member 12. More specifically, the mesh enclosure member 41 can be cloth woven out of thread, knit made of knitted thread, or the like. The mesh enclosure member 41 may be nonwoven cloth made of a plurality of threads crossed each other. The breathability of the mesh enclosure member 41 allows inflow and outflow of air between the area where the grains 42 are confined and outside air. Assuming the enclosure member 12 is not breathable with the closed area containing a considerable amount of air, a collision with the pivoting member (key or massive body) causes expansion of the surface of the enclosure member 12 to exhibit action of air spring. As a result, elastic recovery force is generated, failing to achieve intended effect. If the mesh enclosure member 41 is rough enough to allow grains 42 to pass through, however, the grains 42 can escape outside. Although the grains 42 can travel in arbitrary directions, therefore, the mesh enclosure member 41 is designed such that the grains 42 are unable to pass through openings of the mesh. In a case where the grains 42 are not spherical, as shown in FIG. 4B, the cross-sectional shape varies according to the position to cut In this case, a direction toward which a plane having a minimum cross-sectional shape 42a is faced is regarded as a passing direction 43. As shown in FIG. 4C, therefore, it is preferable that the mesh enclosure member is designed such that dimensions of a mesh opening are smaller than those of the minimum cross-sectional shape 42a, as a guideline, so 25 that the mesh enclosure member 41 is able to prevent the grains 42 from passing through the mesh openings. The mesh enclosure member 41 extends in the direction in which the keys are arranged, however, both ends of the mesh enclosure member 41 situated in the direction of the arranged keys may be covered with the same material as the mesh enclosure member 41 to prevent the grains 42 from flowing away. Alternatively, any different member may be employed to seal the both ends. FIG. 4D shows a concrete example in which a thin film 44 is employed as the enclosure member 12. As the thin film 44, plastic, natural rubber, synthetic rubber or the like can be used. Foamed plastic, foamed rubber or the like can be also used as the thin film 44. The thin film 44 is provided with a plurality of air vents 44a. The air vents 44a may be scattered over the entire surface including the surface at which collisions with the pivoting members (keys or massive bodies) occur. As shown in FIG. 4D, alternatively, the air vents 44a may be scattered, avoiding the surface on which collisions occur. As shown in FIG. 4E, it is preferable that the thin film is designed such that dimensions of the air vent 44a are smaller than those of the minimum cross-sectional shape 42a of each grain, as a guideline, so that the thin film 44 is able to prevent the grains 42 from passing through the air vents 44a. The air vents 44a can be any shape, however, it is preferable that the air vents 44a have sharp edges such as shown triangle and rectangle. Among the air vents 44a having the same cross-sectional area, the air vents 44a having sharp edges are more likely to block the grains 42, for many of the grains 42 are round. As a result, the air vents 44a having sharp edges are likely to prevent the grains 42 from passing through the thin film 44. In 26 addition, even if the grains 42 are lodged in the air vents 44a, clearances on the edges of the air vents 44a ensure breathabillty. The breathability of the thin film 44, in other words, the property of Inflow/outflow of air can be controlled in accordance with the size of the air vents 44a and the number of the air vents 44a. FIG. 4F Illustrates an example in which the base of the "particle bag" is provided with a plurality of air vents. In this case, a base portion 45 having air vents 45a is provided on the horizontal portion 3a of the frame 3 with mount portions 46 being provided between the horizontal portion 3a and the base portion 45. As for this structure, the lower limit stopper 5 shown in FIG. 1 will be taken as an example. The mount portions 46 ensure clearances which allow inflow/outflow of air between the base portion 45 and the key frame 3 (the horizontal portion 3a). The mount portions 46 may be integrated with the base portion 45 or the horizontal portion 3a. Alternatively, the mount portions 46 may be separately configured such as spacer. As the enclosure member 12, the above-described mesh enclosure member 41 or the thin film 44 having the air vents 44a may be employed. Alternatively, a thin film without air vents may be employed. Without using the base portion 45, in FIGS. 1A to 1C, FIG. 2 and FIGS. 3A to 30, a fixing member of the key frame 3 side (or the key frame 3 itself) may be provided with a plurality of air vents equivalent to the air vents 45a. The thin film 44 extends in the direction in which the keys are arranged, however, both ends of the thin film 44 situated in the direction of the arranged keys may be covered with the same material as the thin film 44 to prevent the grains 11 from flowing away. Alternatively, any different 27 member may be employed to seal the both ends. FIG. 5 illustrates a third embodiment of the present invention. The third embodiment is obtained by modifying the first embodiment shown in FIG. 1 such that a conventional elastic action restricting member such as felt is added to the lower limit stopper 5 and the upper limit stopper 6, respectively. In FIG. 5, parts similar to those employed in FIGS. 1A to 1C are given the same numbers as in FIG. 1A to 1C. A felt 51 is provided along with the lower limit stopper 5 which is the "particle bag" to integrally serve as a lower limit stopper. A felt 52 is similarly added to the upper limit stopper 6 which is the "particle bag" to integrally serve as an upper limit stopper. When the white key main body 1 is depressed, the right and left side surfaces of the white key main body 1 collide with both of the lower limit stopper 5 and the felt 51. Resultantly, advantages of the both can be exploited. More specifically, the "particle bag" offers high damping, but is poor in reproducibility in shape (reproducibility of initial position and operating range of the pivoting member) and in tolerance to repeated depressions. Therefore, the reproducibility in initial position and operating range of a key and the tolerance to repeated depressions are made up for by the conventional elastic action restricting member, so that damping of key and operability of the keyboard apparatus are enhanced. When the white key main body 1 is released to return to its initial position, in addition, the white key main body 1 collides with both of the upper limit stopper 6 and the felt 52. On this collision, advantages of both of the upper limit stopper 6 and the felt 52 can be similarly exploited. As shown in FIG. 5, assuming the height of the lower limit stopper 5 28 is h1 with the height of the felt 51 being h2, this embodiment is designed such that it becomes hi