DEVICE FOR ROLLING METAL SHEET MATERIAL AND METHOD FOR ROLLING METAL SHEET MATERIAL Technical Field [OOO 1 ] The present invention relates to a device for rolling a metal sheet material and a method for rolling a metal sheet material. In particular, the present invention relates to a rolling device suitable for thick-plate rolling devices, or roughing rolling devices or finishing rolling devices in a thin-sheet hot rolling device, and capable of increasing the maximum gap between the top and the bottom work rolls and easily applying a strong roll bending force, whereby it is possible to achieve high-response and strong plate, strip-crown and shape controlling function, and a rolling method using this rolling device. The present application claims priority based on Japanese Patent Application No. 201 1-066153 filed in Japan on March 24, 201 1, the disclosure of which is incorporated herein by reference in its entirety. Background Art [0002] Conventionally, rolling devices such as rolling devices 1A and 1 B illustrated in FIG. 12 and FIG. 13 are known, in which back up roll chocks cradle and hold work roll chocks to obtain increased roller gap. However, an increase-bending device is incorporated in an arm portion extending from the back up roll chock for holding the work roll chock, and hence, it is difficult to employ a large-capacity hydraulic cylinder. [0003] For example, a rolling device disclosed in Patent Document 1 is a four-high rolling device having a structure type illustrated in FIG. 16. More specifically, a top work roll chock 3-1 is held by an arm portion connected with a top back up roll chock 4-1. Top increase-bending devices 6-1 and 6-2 that each work to a top work roll 1-1 are assembled to the arm portion so as to achieve a large roller gap. However, this type of rolling device has the following problems. (1) The top increase-bending devices 6-1,6-2 are required to be incorporated in the arm portion of the top back up roll chock 4-1 that cradles the top work roll chock 3-1. Thus, it is difficult to assemble the large-capacity hydraulic cylinder. [0005] (2) If the strong increase-bending force is applied to the top work roll 1-1, the arm portion connected to the top back up roll chock 4- 1 receives a moment acting in the direction in which the arm portion opens outwards. In this case, it is highly possible that the arm portion is pressed on the housing window, and the mill hysteresis increases, causing deteriorated thickness accuracy, or a frictional force between the chock and the window is not in symmetry in terms of right-left direction, which causes the metal sheet to meander during rolling or increases instability of a rolling operation due to occurrence of camber. Thus, it is substantially impossible to use the strong work roll bender. [0006] (3) The top work roll chock 3-1 is brought into contact with the inner surface of the arm portion of the top back up roll chock 4- 1, and the side surface of the arm portion is brought into contact with the inner surface of the housing window. Thus, the metal sheet increasingly wobbles in the rolling direction, which increases the possibility of meandering of the metal sheet or occurrence of camber during rolling. [0007] (4) Spaces are strictly limited, and in particular, complicated arrangement for wires from the back up roll chock is required at the time of exchanging the back up roll. Thus, it is difficult to install a mill stabilizer and load cell for measurement of the force in the rolling direction. [0008] There exists a rolling device such as a rolling device 1 C illustrated in FIG. 14 in which the increase-bending devices 6-1 and 6-2 are incorporated in the bottom work roll chock 3-2 to achieve a large roller gap. 6 For example, Patent Document 2 discloses a rolling device in which the increase-bending device of the work roll is incorporated in the work roll chock. Similarly, Patent Document 3 discloses a rolling device with a roll-cross system. In this rolling device, the increase-bending device is incorporated in the work roll chock. [0009] This type of rolling device has the following problems. (5) At the time of exchanging the work roll, hydraulic pipes need to be detached. To make the detachment easy, it is necessary to employ flexible pipes, which makes it difficult to employ a servo valve for controlling high-response hydraulic pressure. Thus, it is difficult to configure a bending device having high responsiveness. [OO 101 Further, a rolling device is known having a configuration in which an increase-bending device is provided to a project block to obtain a bending device having high responsiveness. [OOl 11 For example, Patent Document 4 discloses a rolling device having a work-roll shift function. In this rolling device, as illustrated in FIG. 15, increase-bending devices 6-1 to 6-4 are incorporated in project blocks 5-1 and 5-2 formed integrally with a housing 9. [OO 121 In the rolling device 1D illustrated in FIG. 15, bottom increase-bending devices 6-3 and 6-4 working to a bottom work roll 1-2 are incorporated in a project block internally protruding from the housing 9. [00 131 However, this type of rolling device has the following problems. (6) This rolling device has a structure in which a force on the work roll acting in the rolling direction is supported by a contact surface on which the project block and the work roll chock are brought into contact with each other. Thus, with the increase in the roll ..*% @, this contact surface becomes smaller. This makes it impossible to appropriately support the work roll chock, and to obtain a large roller gap. [00 141 (7) The capacity of the hydraulic cylinder in the increase-bending device depends on the height of the project block. Thus, if the rolling device includes a project block not having sufficient height, the hydraulic cylinder cannot provide sufficient stroke length, which makes it impossible to obtain the large roller gap. [00 1 51 It should be noted that the increase-bending device represents a hydraulic device that applies a force acting on the work roll chock and in the direction in which the roller gap increases, and is a generic name of a device including a hydraulic cylinder serving as an actuator of the hydraulic device and a piston rod of the hydraulic device. [00 1 61 However, in the present invention, for the sake of simplicity, the increase-bending device represents the hydraulic cylinder serving as the actuator of the increase-bending device and the piston rod of the hydraulic cylinder unless otherwise specified. A force applied to the work roll by the increase-bending device is referred to as the increase-bending force. [00 171 A hydraulic device that applies, to the work roll chock, a force acting in a direction in which the roller gap decreases is referred to as a decrease-bending device. Further, a force applied to the work roll by the decrease-bending device is referred to as the decrease-bending force. Yet further, the decrease-bending device is a generic name of the device including the hydraulic cylinder serving as the actuator of the device and the piston rod of the hydraulic cylinder. In the present invention, for the sake of simplification, the decrease-bending device represents the hydraulic cylinder serving as + the actuator of the decrease-bending device and the piston rod of the decrease-bending device, unless otherwise specified. [00 1 81 It should be noted that, in general, as illustrated in FIG. 12 to FIG. 16, the rolling device for manufacturing the thick steel sheet does not have the decrease-bending device (see Patent Documents 1 to 4). This is because: (A) in the case of thick sheet rolling, a relatively large-diameter work roll is used as compared with a case of thin sheet rolling, and hence, a change in shape of the roll crown is smaller even when the same bending force is applied; and (B) even if a small decrease-bending device is installed at a narrow portion in the vicinity of a relatively large-diameter work roll chock as compared with the thin sheet rolling device, the range of control is narrow, and the machine structure becomes complicated, which results in a low cost performance. Related Art Documents Patent Documents [0019] Patent Document 1 : Japanese Unexamined Patent Application, First Publication NO. HOB-8701 1 Patent Document 2: Japanese Unexamined Patent Application, First Publication NO. S62-220205 Patent Document 3: Japanese Unexamined Patent Application, First Publication NO. H06-198307 Patent Document 4: Japanese Unexamined Patent Application, First Publication NO. H04-520 14 Disclosure of the Invention Problems to be Solved by the Invention [0020] As described above, conventionally, a rolling device having an increased roller gap cannot achieve high responsiveness or accommodate a strong roll bending device. A problem to be solved by the present invention is to provide a rolling device capable of increasing the maximum gap between the top and the bottom work rolls and applying a strong roll bending force, and a rolling method using this rolling device. More specifically, an object of the present invention is to provide a rolling device including a decrease-bending device as with a thin sheet rolling device and capable of: dealing with a steel sheet having a wide range of thickness; increasing the roller gap between the top and the bottom work rolls; easily applying a strong roll bending force; and overcoming the above-described drawbacks that the conventional rolling device has, and a rolling method using this rolling device. Means for Solving the Problems [002 11 The following are the main points of the present invention. (1) A first aspect of the present invention provides a device for rolling a metal sheet material, which includes: a top work roll and a bottom work roll that roll a metal sheet material; a top back up roll and a bottom back up roll that support the top work roll and the bottom work roll, respectively; a top work roll chock and a bottom work roll chock that support the top work roll and the bottom work roll, respectively; a top back up roll chock and a bottom back up roll chock that support the top back up roll and the bottom back up roll, respectively; a housing that accommodates the top work roll chock, the bottom work roll chock, the top back up roll chock, and the bottom back up roll chock, said housing including a pair of first project blocks protruding inwardly that bears a force in a rolling direction acting on the bottom work roll, and further including a housing window that bears a force in the rolling direction acting on the top work roll; a first hydraulic cylinder provided to the pair of first project blocks and including a first piston rod that applies an increase-bending force by way of the top work roll chock to the top work roll; a second hydraulic cylinder provided to the pair of first project blocks and including a second piston rod that applies an increase-bending force by way of the bottom work roll chock to the bottom work roll; a third hydraulic cylinder provided to the top a c k up roll chock and including a third piston rod that applies a decrease-bending force to the top work roll or that brings the top work roll into contact with the top back up roll to generate a roll balance force; and, a fourth hydraulic cylinder including a fourth piston rod that applies a decrease-bending force to the bottom work roll. (2) In the device for rolling a metal sheet material according to (1) described above, the first hydraulic cylinder and the second hydraulic cylinder may be arranged in the pair of first project blocks at positions different from each other in plan view. (3) In the device for rolling a metal sheet material according to (1) or (2) described above, the bottom back up roll chock may be provided with the fourth hydraulic cylinder. (4) In the device for rolling a metal sheet material according to (1) or (2) described above, the housing may further include a pair of second project blocks located below the pair of first project blocks and protruding inwardly from the housing, and the pair of second project blocks may be provided with the fourth hydraulic cylinder. (5) In the device for rolling a metal sheet material according to any one of (1) to (4) described above, the third piston rod may have a top end portion having a first engagement portion, and the top work roll chock may have a second engagement portion that engages with the first engagement portion through movement of the top work roll in a roll axial direction. (6) A second aspect of the present invention provides a method for rolling a metal sheet material using the device for rolling a metal sheet material according to any one of (1) to (5) described above, in which the third piston rod may perform a pull operation to generate a roll balance force when rolling is performed with a roller gap exceeding a stroke of the first hydraulic cylinder. (7) A third aspect of the present invention provides a method for rolling a metal sheet material using the device for rolling a metal sheet material according to any one of (1) to (5) described above, in which, in a case where rolling is performed with a roller gap that falls within a stroke of the first hydraulic cylinder, the method includes: a first process of, prior to start of rolling, causing an increase-bending force and a decrease-bending force to act on the top work roll and the bottom work roll to cause a roll bending force &-responding to a roll balance force to act as a resultant force on the top work roll and the bottom work roll; a second process of, after the first process, increasing the increase-bending force while changing the decrease-bending force to be a decrease-bending force during rolling so as to maintain the resultant force to be the roll balance force; a third process of, at start of rolling, changing the increase-bending force while maintaining the decrease-bending force to cause a roll bending force corresponding to a roll bending force during rolling to act as a resultant force on the top work roll and the bottom work roll; a fourth process of performing rolling while maintaining the roll bending force during rolling; a fifth process of, at completion of rolling, changing the increase-bending force while maintaining the decrease-bending force to cause a roll bending force corresponding to the roll balance force to act as a resultant force on the top work roll and the bottom work roll, and in this state, ending rolling of the metal sheet material; and a sixth process of, thereafter, decreasing the decrease-bending force and the increase-bending force so as to maintain the roll balance force. (8) In the method for rolling a metal sheet material according to (7) described above, continuous measurement may be performed on at least one of hydraulic pressure in the third hydraulic cylinder, hydraulic pressure in a hydraulic pipe connected with the third hydraulic cylinder, hydraulic pressure in the fourth hydraulic cylinder, and hydraulic pressure in a hydraulic pipe connected with the fourth hydraulic cylinder, and based on a measurement result, the increase-bending force may be controlled such that the roll bending force acting as a resultant force on the top work roll chock and the bottom work roll chock becomes a predetermined value. (9) A fourth aspect of the present invention provides a method for rolling a metal sheet material using the device for rolling a metal sheet material according to any one of (1) to (5) described above, in which, a roll balance force for the top work roll is applied through a pull operation performed by the third piston rod in a case where rolling is performed with a roller gap exceeding a stroke of the first hydraulic cylinder during rolling, and then, in a case where rolling is performed with a roller gap that falls within a stroke of the first hydraulic cylinder, the method includes: a first process of, prior to start @rolling, causing an increase-bending force and a decrease-bending force to act on the top work roll and the bottom work roll to cause a roll bending force corresponding to a roll balance force to act as a resultant force on the top work roll and the bottom work roll; a second process of, after the first process, increasing the increase-bending force while changing the decrease-bending force to be a decrease-bending force during rolling so as to maintain a resultant force to be the roll balance force; a third process of, at the start of rolling, changing the increase-bending force while maintaining the decrease-bending force to cause a roll bending force corresponding to a roll bending force during rolling to act as a resultant force on the top work roll and the bottom work roll; a fourth process of performing rolling while maintaining the roll bending force during rolling; a fifth process of, at completion of rolling, changing the increase-bending force while maintaining the decrease-bending force to cause a roll bending force corresponding to the roll balance force to act as a resultant force on the top work roll and the bottom work roll, and in this state, ending rolling of the metal sheet material; and a sixth process of, thereafter, decreasing the decrease-bending force and the increase-bending force so as to maintain the roll balance force. (10) In the method for rolling a metal sheet material according to (9) described above, continuous measurement may be performed on at least one of hydraulic pressure in the third hydraulic cylinder, hydraulic pressure in a hydraulic pipe connected with the third hydraulic cylinder, hydraulic pressure in the fourth hydraulic cylinder, and hydraulic pressure in a hydraulic pipe connected with the fourth hydraulic cylinder, and based on a measurement result, the increase-bending force may be controlled such that the roll bending force acting as a resultant force on the top work roll chock and the bottom work roll chock becomes a predetermined value. Effects of the Invention [0022] According to the rolling device and the rolling method according to (1) to (10) described above, the maximum gap between the top and the bottom work rolls can be &eased, and even in the case where the decrease-bending device having low responsiveness is provided, this is compensated for by the increase-bending device having high responsiveness and provided to the pair of first project blocks, whereby it is possible to obtain a high-response and strong plate, strip-crown and shape controlling function. Thus, it is possible to obtain favorable plate, strip crown and shapes even if there exist external disturbances such as thickness of the material on the input side or temperatures of the rolling material, which varies during rolling, whereby it is possible to significantly improve product quality and production yield. Further, the increase-bending force and the decrease-bending force are applied to the top work roll to achieve the strong plate, strip-crown and shape controlling function, and further, a multi-stage hydraulic cylinder is employed for the top-work-roll decrease-bending device to achieve generation of the roll balance force, whereby it is possible to largely increase the roll gap. In other words, with only one rolling device, it is possible to deal with various applications ranging from blooming roll with a large thickness to hot-roll thin sheet roll for which precise plate, strip crown and shape control is required. Yet further, the force in the rolling direction acting on the top work roll chock is always borne by the housing window, whereby it is possible to stably support the top work roll chock. Yet further, the top and the bottom increase-bending devices can be incorporated in the pair of first project blocks. This makes it possible to achieve the strong bending device having the large capacity and large stroke. Yet further, the increase-bending device is incorporated in the pair of first project blocks, which makes it possible to fix the hydraulic pipes and employ servo valves. With this configuration, it is possible to control the increase-bending force in a high response manner. Yet further, even if the decrease-bending device having low responsiveness is used, it is possible to control the roll bending force in a high response manner with bperation of the increase-bending device having high responsiveness. This makes it possible to largely improve the product quality and yield in terms of rolling. Brief Description of the Drawings [0023] FIG. 1 is a side view illustrating a structure of a rolling device according to an embodiment of the present invention. FIG. 2 is a diagram illustrating a connection structure between a top work roll chock and a top decrease-bending device. FIG. 3A is a sectional view illustrating a first mode of an engagement relationship between a first engagement portion of a third piston rod of a third hydraulic cylinder and a second engagement portion of a top work roll chock. FIG. 3B is a sectional view illustrating a second mode of an engagement relationship between a first engagement portion of a third piston rod of a third hydraulic cylinder and a second engagement portion of a top work roll chock. FIG. 3C is a sectional view illustrating a third mode of an engagement relationship between a first engagement portion of a third piston rod of a third hydraulic cylinder and a second engagement portion of a top work roll chock. FIG. 4 is a perspective plan view illustrating an example of arrangement of top and bottom increase-bending devices. FIG. 5 is a perspective plan view illustrating an example of arrangement of top and bottom increase-bending devices. FIG. 6 is a side view illustrating another example of a structure of a rolling device according to an embodiment of the present invention. FIG. 7 is a diagram illustrating an example of an operation flow of a rolling method according to an embodiment of the present invention. FIG. 8 is a diagram illustrating a change in time series of a roll bending force and other parameters associated with the operation flow in FIG. 7. FIG. 9 is a diagram illustrating a change in time series of a roll bending force and other parameters in the case where the responsiveness of a decrease-bending device is significantly low. FIG. 10 is a diagram illustrating another example of an operation flow of a rolling method according to an embodiment of the present invention. FIG. 11 is a diagram illustrating a change in time series of a roll bending force and other parameters associated with the operation flow in FIG. 10. FIG. 12 is a side view illustrating a structure of a rolling device 1A according to a conventional art. FIG. 13 is a side view illustrating a structure of a rolling device 1B according to a conventional art. FIG. 14 is a side view illustrating a structure of a rolling device 1 C according to a conventional art. FIG. 15 is a side view illustrating a structure of a rolling device 1D according to a conventional art. FIG. 16 is a side view illustrating a structure of a rolling device 1E according to a conventional art. Embodiments of the Invention [0024] Hereinbelow, a rolling device according to the present invention based on the findings described above and a rolling method using this rolling device will be described with reference to FIG. 1 through FIG. 16. FIG. 1 is a side view illustrating an example of a structure of a rolling device 1 according to an embodiment of the present invention. As illustrated in FIG. 1, the rolling device 1 according to an embodiment of the present invention includes a top work roll chock 3-1, a bottom work roll chock 3-2, a top back up roll chock 4-1, a bottom back up roll chock 4-2, and a housing 9 containing these roll chocks. The housing 9 has a pair of first project blocks 5-1 and 5-2 (in other words, a first project block provided on the side *ere a metal sheet material is inputted and a first project block provided on the output side) formed integrally with the housing, thereby forming a housing window 12. The top work roll chock 3- 1 supports a top work roll 1-1 for rolling a metal sheet material, and the bottom work roll chock 3-2 supports a bottom work roll 1-2 for rolling the metal sheet material. Further, the top back up roll chock 4-1 supports a top back up roll 2-1 disposed above the top work roll 1-1, and the bottom back up roll chock 4-2 supports a bottom back up roll disposed below the bottom work roll 1-2. [0025] The pair of the first project blocks 5-1 and 5-2 are formed integrally so as to protrude inwardly from the housing 9. The pair of the first project blocks 5-1 and 5-2 are provided with top increase-bending devices 6-1 and 6-2 that each apply an increase-bending force through the top work roll chock 3-1 to the top work roll 1-1, and are provided with bottom increase-bending devices 6-3 and 6-4 that each apply an increase-bending force through the bottom work roll chock 3-2 to the bottom work roll 1-2. More specifically, the top increase-bending devices 6-1 and 6-2 are each formed by a first hydraulic cylinder serving as an actuator of the device 6-1,6-2 and a piston rod (first piston rod) of the first hydraulic cylinder. The first hydraulic cylinder is incorporated in the pair of the first project blocks 5-1 and 5-2, and is provided such that an end portion of the first piston rod protrudes from the top surface of each of the pair of the first project block 5-1,5-2 and comes into contact with the top work roll chock 3-1. Further, the bottom increase-bending devices 6-3 and 6-4 are each formed by a second hydraulic cylinder serving as an actuator of the device 6-3,6-4 and a piston rod (second piston rod) of the second hydraulic cylinder. The second hydraulic cylinder is incorporated in the pair of the first project blocks 5-1 and 5-2, and is provided such that an end portion of the second piston rod protrudes from the bottom surface of each of the pair of the first project blocks 5-1 and 5-2 and comes into contact with the bottom work roll chock 3-2. 16261 Further, the rolling device 1 according to this embodiment includes top decrease-bending devices 7-1 and 7-2 provided in the top back up roll chock 4-1 and having a function of applying the decrease-bending force through the top work roll chock 3-1 to the top work roll 1-1 and a function of causing a pulling-up force (roll balance force) for bringing the top work roll 1-1 into contact with the top back up roll 2-1. The top decrease-bending devices 7-1 and 7-2 having the two hctions are formed by a third hydraulic cylinder serving as an actuator of the device 7-1,7-2 and a piston rod (third piston rod) of the third hydraulic cylinder. The third hydraulic cylinder is provided in the top back up roll chock 4-1. The third piston rod includes a top end shaped so as to engage with the top work roll chock 3-1. [0027] Further, the rolling device 1 according to this embodiment includes bottom decrease-bending devices 7-3 and 7-4 that each apply the decrease-bending force through the bottom work roll chock 3-2 to the bottom work roll 1-2. The bottom decrease-bending devices 7-3 and 7-4 are each formed by a fourth hydraulic cylinder serving as an actuator of the device 7-3,7-4 and a piston rod (fourth piston rod) of the fourth hydraulic cylinder. The fourth hydraulic cylinder is provided to the bottom back up roll chock 4-2, or is incorporated in a pair of second project blocks 5-3 and 5-4, which will be described later. The fourth piston rod is provided such that a top end thereof is brought into contact with the bottom work roll chock 3-2. [0028] The configuration in which a project block has an increase-bending device, and top and bottom back up roll chocks each have a decrease-bending device is commonly found in hot-rolling finish rolling devices. However, with this configuration, the force acting in the rolling direction and occurring during rolling has to be supported by the contact surface on which the surface on the top stream side of the project block provided on the output side of the metal sheet material is brought into contact with the work roll chock. Thus, with the increase in the roller gap, the center of the rotational moment of & top work roll deviates upward from the contact surface, and the area of the contact surface that supports the force in the rolling direction decreases. For these reasons, in the top work roll chock, even if the height of the pair of the project blocks is increased, the contact surface between the surface on the top stream side of the project block and the work roll chock decreases with the increase in the roller gap, and the position of the top work roll chock becomes unstable, which makes it impossible to increase the roller gap. [0029] A rolling device 1E illustrated in FIG. 16 improves the above-described points, and is frequently found in thick-sheet finishing rolling devices. In the rolling device 1 E, the top back up roll chock 4-1 includes an arm that cradles the top work roll chock 3-1. With this type of rolling device, the top back up roll chock 4-1 cradles the top work roll chock 3-1, and hence, the top work roll 1-1 ascends as the top back up roll 2-1 elevates, which makes it possible to increase the roller gap. [0030] However, the rolling device having this structure does not have sufficient space for the increase-bending device and the decrease-bending device used for the top work roll 1 - 1. Thus, in general, as illustrated in FIG. 16, for the top work roll 1 - 1, only the small-capacity increase-bending devices 6-1 and 6-2 are provided between the top back up roll chock 4-1 and the top work roll chock 3-1, and no decrease-bending device is provided for the top work roll 1 - 1. This results in a drawback of limited shape-controlling ability. [003 11 Further, the force in the rolling direction acting on the top work roll 1-1 is received by the contact surface on the downstream side in the rolling direction between the top back up roll chock 4-1 and the top work roll chock 3-1. Further, this force is finally received by the contact surface between the outer surface of the arm on the downstream side in the rolling direction of the top back up roll chock 4-1 and the inner side of the housing window 12. However, the contact surface on the downstream side in the rolling direction has limited space, and it is difficult to install a load cell that can jh,iciently support the top work roll chock 3-1. Further, a wobble remains on the contact surface between the top back up roll chock 4- 1 and the housing window 12. Yet further, a stabilizer or load cell is required for each back up roll chock, and arrangement of wires necessary for activating these units significantly deteriorates workability. [0032] In view of the circumstances described above, the rolling device 1 according to this embodiment includes the pair of the first project blocks 5-1 and 5-2 protruding inwardly from the housing 9 and disposed at positions shifted downward with respect to a pass line, as illustrated in FIG. 1. More specifically, unlike the conventional configuration illustrated in FIG. 15, the pair of the first project blocks 5-1 and 5-2 are disposed at positions that are not vertically symmetric with respect to the pass line. Further, the top work roll chock 3-1 is shaped such that a base portion adjacent to the portion located between the pair of the first project blocks 5-1 and 5-2, in other words, the top side portion corresponding to the width of the housing window 12 (housing window width) is set higher instead of increasing the height of the portion located between the pair of the first project blocks 5-1 and 5-2. [0033] With the rolling device 1 according to this embodiment having such a shape, the force in the rolling direction such as a component of the offset force acting on the top work roll 1-1, in other words, the force in the rolling direction acting on the body portion of the top work roll 1-1 from the metal sheet (plate) material 10 or top back up roll 2-1 is borne by the contact surface between the top side portion of the top work roll chock 3-1 corresponding to the housing window width and the housing window 12 located above the pair of the first project blocks 5-1 and 5-2. [0034] With this structure, the area of the surface on which the top work roll chock 3-1 is brought into contact with the housing window 12 remains unchanged even if a drafting device 1 1 of the rolling device 1 is operated to increase the roller gap. Thus, the position &the top work roll chock 3-1 can be stably maintained regardless of the amount of the roller gap. [0035] Further, the rolling device 1 according to this embodiment includes the pair of the first project blocks 5-1 and 5-2 inwardly protruding fiom the housing 9 and including the top increase-bending devices 6-1 and 6-2 that apply the increase-bending force to the top work roll 1-1, and the bottom increase-bending devices 6-3 and 6-4 that apply the increase-bending force to the bottom work roll 1-2. [0036] This configuration eliminates the need for detaching the hydraulic pipes of the increase-bending device every time the work roll is exchanged, and allows using fixed pipes. This makes it possible to employ accurate servo valves, thereby obtaining a high-responsive increase-bending device. [0037] Further, the top decrease-bending devices 7-1 and 7-2 are formed by the third hydraulic cylinder incorporated in the top back up roll chock 4-1 and the third piston rod of the third hydraulic cylinder. The top end of the third piston rod of the third hydraulic cylinder is operated to move in the roll axis direction at the time of exchanging the roll so as to be able to be connected with the top work roll chock 3-1, thereby applying the roll balance force. [003 81 With this configuration, the roller gap can be increased regardless of the stroke of the increase-bending device, and it is possible to apply the strong decrease-bending force in the case where a plate, strip crown control is necessary for a steel plate with a thickness of approximately 100 rnrn or less. This makes it possible for only one rolling device 1 to deal with various rolling applications ranging from rolling of a roll material with a thickness of over 800 mm to rolling with a precise plate, strip crown control during the thick-sheet rolling with a thickness of 100 mm or less. [0039] & Next, an example of a structure of the connection portion will be described in detail. This example shows merely one embodiment, and does not limit the specific structure of the present invention. FIG. 2 is a plan view of the top work roll 1 - 1 and the top work roll chock 3- 1, and in the drawing, the near side shows the drive side, while the far side shows the work side. At each of the right and the left on the top surface of the roll chock 3-1 for the top work roll 1-1, there is formed a groove 3 1 having a reversed T-shape in cross-section and engaged with the top end of the third piston rod of the third hydraulic cylinder constituting the top decrease-bending device 7-1,7-2 as a first engagement portion. The groove 3 1 opens on the drive side, and extends up to the vicinity of the center of the chock. When a roll set is inserted into the rolling device 1, the work roll having both ends each set with the roll chock from the operation side is delivered through a rail or other tool to a predetermined position in the housing of the rolling device 1. At this time, the piston rod of the hydraulic cylinder serving as the decrease-bending device is retracted in the top back up roll chock 4-1, so that this piston rod does not obstruct the delivery. Next, the groove 3 1 of the top work roll chock 3-1 is positioned immediately in front of an expanded portion (second engagement portion) of the third piston rod of the third hydraulic cylinder to be engaged with the groove 3 1. At this position, the piston rod is descended to a predetermined position. Then, as illustrated in FIG. 3C, the inner surface of the groove 3 1 is kept in a position where it does not come into contact with the external surface of the third piston rod, and the expanded portion of the third piston rod is moved into the groove 3 1 from the groove opening portion on the drive side. Through these procedures, the first engagement portion and the second engagement portion are engaged with each other. [0040] FIG. 3A is a diagram illustrating a state where the expanded portion formed at the top end of the third piston rod of the third hydraulic cylinder of the top decrease-bending device 7-1,7-2 is engaged with the groove 3 1. More specifically, FIG. 3A illustrates a state where, with the pulling-up operation made by the third hydraulic ainder, the third piston rod is pulled upward and is brought into contact with the top surface of the groove 3 1 of the top work roll chock 3- 1, thereby applying a lifting force ~RB. [004 11 The lifting force fRB acts as the roll balance force. Thus, without relying on the stroke of the first piston rod of the top increase-bending devices 6-1 and 6-2 provided to the pair of the first project blocks 5-1 and 5-2, it is possible to raise the top work roll 1-1 together with the top back up roll 2- 1. This makes it possible to easily obtain the large roller gap. [0042] FIG. 3B is a diagram illustrating a state where the top decrease-bending force is applied in the case of controlling plate, strip crown and shapes in such a case as finishing rolling in the thin-sheet hot rolling. More specifically, with the third hydraulic cylinder of the top decrease-bending device 7-1,7-2, the third piston rod extends downward to apply a pressing force fDc to the bottom surface of the groove 3 1 of the top work roll chock 3- 1, thereby applying the decrease-bending force. [0043] FIG. 3C is a diagram illustrating a non-contact state where the groove 3 1 is brought into engagement with the third piston rod at the time of exchanging rolls as described above, or this engagement is canceled. [0044] In the example described above, the groove 3 1 is formed in the top work roll chock 3-1, and the expanded portion to be engaged with the groove 3 1 is formed in the top decrease-bending device 7-1,7-2. However, the present invention is not limited to this mode. For example, it may be possible that the groove 3 1 is formed in the top decrease-bending device 7-1,7-2, and the expanded portion to be engaged with the groove 3 1 is formed in the top work roll chock 3-1. [0045] j) It should be noted that, in the rolling device 1 according to this embodiment, the force in the rolling direction applied to the body portion of the bottom work roll 1-2 is borne by the contact surface between the bottom work roll chock 3-2 and the project block (5-1 or 5-2) provided on the output side. Thus, in the rolling device 1 according to this embodiment illustrated in FIG. 1, a portion of the bottom work roll chock 3-2 located between the pair of the first project blocks 5-1 and 5-2 is raised. [0046] Further, since the roller gap is adjusted mainly by moving upward and downward the top work roll chock 3-1, the amount of travel of the bottom work roll chock 3-2 is small in the vertical direction. Thus, the positions of the bottom work roll 1-2 remain stable even if the roller gap is increased. [0047] FIG. 4 is a plan sectional view illustrating an example of arrangement of the top and bottom increase-bending devices 6-1 to 6-4. In other words, FIG. 4 is a sectional view at the height of the pass line of the pair of the first project blocks 5-1 and 5-2. In the rolling device 1 according to this embodiment, it is desirable to arrange the top and bottom increase-bending devices 6-1 to 6-4 so as to be positionally shifted to each other on the plan sectional view of the pair of the first project blocks 5-1 and 5-2. For example, as illustrated in FIG. 4, it is desirable to arrange the top increase-bending devices 6-1 and 6-2 and the bottom increase-bending devices 6-3 and 6-4 so as to be positionally shifted in the axial direction of the work roll 1-2. With this arrangement, the top and bottom increase-bending devices 6-1 to 6-4 do not interfere with each other within the pair of the first project blocks 5-1 and 5-2. In other words, the first hydraulic cylinder and the second hydraulic cylinder, which are incorporated in each of the first project blocks 5-1 and 5-2, do not interfere with each other. This makes it possible to increase the capacity of the first hydraulic cylinder and the second hydraulic cylinder, increase the stroke of each of the first piston rod and the second piston rod, and increase the amount of operation of the increase bending. [0048] & It should be noted that, in FIG. 4, the bottom increase-bending devices 6-3 and 6-4 are formed by two second hydraulic cylinders located on the input side and the output side. However, a similar effect can be obtained by using one second hydraulic cylinder for each of the bottom increase-bending devices 6-3 and 6-4 and arranging them at different positions in the axial direction of the bottom work roll 1-2 to prevent the second hydraulic cylinder fiom interfering with the first hydraulic cylinder. [0049] FIG. 5 is a plan sectional view illustrating an example of arrangement of the top and bottom increase-bending devices 6-1 to 6-4. In other words, FIG. 5 is a sectional view at the height of the pass line of the pair of the first project blocks 5-1 and 5-2. As illustrated in FIG. 5, the first hydraulic cylinder and the second hydraulic cylinder may be arranged so as to be positionally shifted in the rolling direction. With this arrangement, the first hydraulic cylinder and the second hydraulic cylinder do not interfere with each other. Thus, it is possible to increase the capacity of the first hydraulic cylinder and the second hydraulic cylinder, increase the stroke of each of the first piston rod and the second piston rod, and increase the amount of operation of bending. These are descriptions of the structure of the rolling device 1 according to this embodiment mainly fiom the viewpoint of increasing the roller gap, which is one of the problems to be solved. [0050] Next, with this structure, a description will be made showing that the strong roll bending force can be easily applied, which is another problem to be solved. FIG. 12 and FIG. 13 illustrate rolling devices 1 A and 1 B according to conventional arts, each of which has an increased roller gap. However, these rolling devices 1A and 1B cannot apply a strong roll bending force. This is because these rolling devices have a structure in which the top increase-bending devices 6-1 and 6-2 are incorporated in the arm portion protruding downward from the top back up roll chock 4-1, which prevents the large-capacity and large-stroke top increase-bending devices 6-1 and 6-2 from being installed. Further, dhese rolling devices 1A and 1B have the arm portion extending from the top back up roll chock 4-1, and hence, the top decrease-bending devices 7-1 and 7-2 are required to be installed at the space shifted to the axial center of the roll. Thus, interference with the bearing for the top back up roll 2-1 occurs, which makes it impossible to dispose the large-capacity and large-stroke top decrease-bending devices 7-1 and 7-2. [005 11 Further, as illustrated in FIG. 1, in the rolling device 1 according to this embodiment, the large-capacity and large-stroke top increase-bending devices 6-1 and 6-2 can be disposed in the pair of the first project blocks 5-1 and 5-2 protruding inwardly from the housing 9 of the rolling device 1. Further, unlike the rolling devices 1A and 1B illustrated in FIG. 12 and FIG. 13, in the rolling device 1 according to this embodiment, the top back up roll chock 4-1 does not have the arm portion. This allows the large-capacity and large-stroke top decrease-bending devices 7-1 and 7-2 to be disposed at positions of the top back up roll chock 4-1 that do not interfere with the bearing for the top back up roll 2-1, whereby it is possible to apply the large decrease-bending force to the top work roll 1-1. [0052] More specifically, according to the rolling device 1 of this embodiment having a structure in which, together with the positions of the pair of the first project blocks 5-1 and 5-2, the force in the rolling direction applied to the body portion of the top work roll 1-1 is received by the contact surface between the top work roll chock 3-1 and the housing window 12, it is possible to increase the roller gap and apply the strong roll bending force. Further, with this structure, it is possible to eliminate the need for detaching the hydraulic pipes of the increase-bending device every time the work rolls are exchanged. This makes it possible for the increase-bending devices 6-1 to 6-4 to be connected with hydraulic control valves through fixed hydraulic pipes and employ servo valves used for high-response hydraulic control. Thus, it is possible to obtain the high-response increase-bending device. [0053] FIG. 6 is a side view illustrating a rolling device 1 ' according to a modification example of this embodiment. The rolling device 1 ' illustrated in FIG. 6 has the same top roll systems as those illustrated in FIG. 1 and different bottom roll systems from those illustrated in FIG. 1. In the rolling device illustrated in FIG. 1, the bottom back up roll chock 4-2 has the bottom decrease-bending devices 7-3 and 7-4 that apply the decrease-bending force to the bottom work roll 1-2. On the other hand, in the rolling device 1 ' illustrated in FIG. 6, the bottom decrease-bending devices 7-3 and 7-4 are disposed to a pair of second project blocks 5-3 and 5-4 located below the pair of the first project blocks 5-1 and 5-2. [0054] If the bottom decrease-bending devices 7-3 and 7-4 are disposed to the bottom back up roll chock 4-2 as in the rolling device 1 illustrated in FIG. 1, the hydraulic pipes of the decrease-bending device are required to be detached when the bottom back up roll 2-2 is exchanged. In other words, it is highly likely that small foreign substances enter the hydraulic pipes at the time of detachment. Thus, in general, it is difficult to employ a servo valve for the high-response hydraulic control, and further, it may be necessary to partially employ flexible pipes. Accordingly, the roll bending device has reduced responsiveness as compared with the case where the fixed pipes or servo valves are employed. [0055] On the other hand, according to the rolling device 1' illustrated in FIG. 6, it is possible to overcome the above-described problem occurring at the time of exchanging the bottom back up roll 2-2. This is because servo valves for high-response hydraulic control can be used for the hydraulic pipes of the bottom decrease-bending devices 7-3 and 7-4 provided to the pair of the second project blocks 5-3 and 5-4, whereby it is possible to eliminate the need for using the flexible pipes. Thus, the bottom back up roll 2-2 can be easily exchanged, and the high-response roll bending device can be obtained. [0056] Next, a rolling method according to this embodiment will be described. In the case where the top decrease-bending devices 7-1 and 7-2 are provided to the top back up roll chock 4-1 as illustrated in FIG. 1 and FIG. 6, the hydraulic pipes of the top decrease-bending devices 7-1 and 7-2 are required to be detached at the time of exchanging the top back up roll 2-1, and hence, it is highly likely that small foreign substances enter the hydraulic pipes at the time of exchange. Thus, in general, it is relatively difficult to employ the servo valve for high-response hydraulic control. Further, in order to easily detach the pipes, it is necessary to connect the hydraulic-control valves through detachable hydraulic pipes having flexibility such as flexible pipes. However, in the case where the detachable hydraulic pipes having flexibility such as flexible pipes are employed, this flexibility may absorb or alleviate the variation of the hydraulic pressure. [0057] Thus, in the case where the top decrease-bending devices 7-1 and 7-2 are provided to the top back up roll chock 4-1, the responsiveness of the roll bending device reduces as compared with the case where the fixed pipes or servo valves are used. Incidentally, the decrease-bending force cannot be applied at idle time when the rolling load is not applied. Thus, when the decrease-bending force is applied, it is necessary to rapidly complete setting the decrease-bending force by the start of rolling from the idle state in which the roll balance force is applied, and rapidly turn back into the roll balance state at the time of completion of the rolling. Thus, if the change in the roll bending force is controlled by the decrease-bending device having reduced responsiveness, a predetermined decrease-bending force is not applied at the tail end of the roll material, possibly causing malformation of the roll material. [OOSS] The rolling method according to this embodiment solves the above-described problems. More specifically, the rolling method uses the rolling device 1, 1 ' according to this embodiment including the top back up roll chock 4-1 provided with the top &crease-bending devices 7-1 and 7-2, and solves the above-described problems that may occur in the rolling device 1, 1 ' . As described above, in the rolling device 1, 1 ' including the top back up roll chock 4-1 provided with the top decrease-bending devices 7-1 and 7-2, the responsiveness of the decrease-bending device may deteriorate. However, the rolling device 1, 1 ' according to this embodiment has a structure in which the pair of the first project blocks 5-1 and 5-2 protruding inwardly from the housing 9 includes the top increase-bending devices 6-1 and 6-2, thereby obtaining the top increase-bending device having the large capacity and large stroke. [0059] It is not necessary to detach the hydraulic pipes of the increase-bending device every time the rolls are exchanged, and hence, the fixed hydraulic pipes or servo valves can be used, which makes it possible to obtain the increase-bending device having the high-responsiveness. The rolling method according to this embodiment uses the increase-bending device having high responsiveness for changing the roll bending force at the time of starting rolling and at the time of finishing rolling, in the case where the decrease-bending force is applied to the work roll to control the plate, strip crown and shapes, thereby compensating for the responsiveness of the decrease-bending device. [0060] FIG. 7 is a diagram illustrating an example of an operation flow of the rolling method according to this embodiment. More specifically, FIG. 7 is a diagram illustrating an operation flow performed by the increase-bending device having the high responsiveness and an operation flow performed by the decrease-bending device having relatively low responsiveness as compared with the increase-bending device. FIG. 8 illustrates a change in time series of the roll bending force to one roll material or other parameters in this rolling method. FIG. 8 indicates, from top to bottom, a change in time series of rolling load, output from the increase-bending device, output from the decrease-bending device, and work-roll bending force serving as the resultant a r c e of these forces. Below, a description will be made with reference to FIG. 7 and FIG. 8. [006 11 First, prior to start of rolling, calculation is made to obtain a setting value FR of the work-roll bending force during rolling that corresponds to a roll material to be next rolled. In this example, it is assumed that the FR is a negative value, in other words, is calculated as the decrease-bending force. Note that, in this embodiment, it is assumed that the increase-bending force (force acting in the increase direction in which roll is opened) is a positive value and the decrease-bending force (force acting in the decrease direction in which the roll is pressed) is a negative value. [0062] [First Step] Prior to the start of rolling, both the increase-bending force and the decrease-bending force are activated, so that a roll bending force on the increase side corresponding to a roll balance force FB acts as a resultant force on the work roll chock. In other words, at the idle time before rolling, IB + DB acts as the roll balance force FB (>O), where IB (>O) is the output from the increase-bending device and DB (O) to the roll bending force FR (O), where IB (>O) is the output from the increase-bending device and DB (O) to the roll bending force FR (