A crank driving apparatus comprising a drive-amount changing mechanism, the mechanism including a first supporting member attached to a driving shaft so as to be unrotatable relative to the driving shaft, a second supporting member attached to the first supporting member so as to be unrotatable relative to the first supporting member, an eccentric shaft supported by the second supporting member, and a coupling member rotatably supported by the second supporting member via the eccentric shaft and coupled to an object that is to be driven, wherein the second supporting member is disposed in such a manner that an attachment position of the second supporting member relative to the first supporting member is adjustable in an eccentric direction in which an axis of the eccentric shaft is positioned eccentric to an axis of the driving shaft, wherein the attachment position of the second supporting member relative to the first supporting member in the eccentric direction determines an eccentric distance of the axis of the eccentric shaft from the axis of the driving shaft, and wherein an amount of drive of the crank driving apparatus is changed by changing the eccentric distance as a result of changing the attachment position, the drive-amount changing mechanism comprising: a supporting portion disposed so as to be secured to one of the first supporting member and the second supporting member; and a position-adjusting member supported by the supporting portion and disposed so as to touch a surface of the other one of the first supporting member and the second supporting member, the surface extending in a direction crossing the eccentric direction, wherein a rotation of the position-adjusting member relative to the supporting portion changes a distance from the supporting portion to a contact position at which the position-adjusting member touches the other supporting member and changes positions of the first supporting member and the second supporting member relative to each other in the eccentric direction, characterized in that the supporting portion is a shank member inserted into an oblong hole and disposed so as to extend parallel to the axis of the driving shaft from a contact surface of the one supporting member that touches the other supporting member, the oblong hole being formed in the other supporting member and extending in a direction parallel to the eccentric direction, wherein the other supporting member has two contact surfaces opposing in a direction parallel to the eccentric direction across the oblong hole, and wherein the position-adjusting member is a disk-shaped member having such a diameter as to be capable of touching the two contact surfaces of the other supporting member at the same time, the position-adjusting member is rotatably supported by the supporting portion in such a manner that the supporting portion is fitted to and inserted into a portion of the position-adjusting member positioned eccentric to a center of the position-adjusting member, and the position-adjusting member touches the contact surfaces of the other supporting member at two portions on an outer peripheral surface opposing in a direction parallel to the eccentric direction while being supported by the supporting portion. a supporting portion disposed so as to be secured to one of the first supporting member and the second supporting member; and a position-adjusting member supported by the supporting portion and disposed so as to touch a surface of the other one of the first supporting member and the second supporting member, the surface extending in a direction crossing the eccentric direction, wherein a rotation of the position-adjusting member relative to the supporting portion changes a distance from the supporting portion to a contact position at which the position-adjusting member touches the other supporting member and changes positions of the first supporting member and the second supporting member relative to each other in the eccentric direction, characterized in that the supporting portion is a shank member inserted into an oblong hole and disposed so as to extend parallel to the axis of the driving shaft from a contact surface of the one supporting member that touches the other supporting member, the oblong hole being formed in the other supporting member and extending in a direction parallel to the eccentric direction, wherein the other supporting member has two contact surfaces opposing in a direction parallel to the eccentric direction across the oblong hole, and wherein the position-adjusting member is a disk-shaped member having such a diameter as to be capable of touching the two contact surfaces of the other supporting member at the same time, the position-adjusting member is rotatably supported by the supporting portion in such a manner that the supporting portion is fitted to and inserted into a portion of the position-adjusting member positioned eccentric to a center of the position-adjusting member, and the position-adjusting member touches the contact surfaces of the other supporting member at two portions on an outer peripheral surface opposing in a direction parallel to the eccentric direction while being supported by the supporting portion.