A rear cushion device to be mounted between a vehicle main body and a rear suspension arm of a straddle-type vehicle and comprising a spring for absorbing shock and a damper configured to attenuate vibrations of the spring, wherein the damper comprises: a cylinder; a piston provided in the cylinder; a rod having one axial end side connected to the piston in the cylinder and the other axial end side extending from the cylinder toward the other axial side; a cylinder-side mounting part provided at one axial end side of the cylinder and provided to mount the rear cushion device between the vehicle main body and the rear suspension arm; and a rod-side mounting part provided at the other axial end side of the rod and provided to mount the rear cushion device between the vehicle main body and the rear suspension arm, characterized in that the rod-side mounting part comprises: an inner member having a cylindrical shape, fixed to the other axial end side of the rod coaxially with the rod, and having an outer peripheral surface on which a screw is formed; and an outer member mounted to an outer peripheral side of the inner member, wherein the outer member comprises: a tubular part having a tubular shape, and having an inner peripheral surface on which a screw to be screwed with the screw of the inner member is formed so that, by adjusting the screwing amount of the inner member into the tubular part, an entire length of the rear cushion device can be adjusted; and a coupling part integrally joined to the tubular part of the outer member for coupling the outer member to the vehicle main body or the rear suspension arm; the rod-side mounting part comprises an adjustment/fixing switching mechanism capable of switching a state in which a screwing amount of the inner member into the tubular part can be adjusted by rotating the inner member relative to the tubular part and a state in which the screwing amount of the inner member into the tubular part is fixed by fixing the inner member so as not to be rotatable relative to the tubular part; and the adjustment/fixing switching mechanism comprises: a slit formed in a peripheral wall of the tubular part and configured to enable a diameter of the tubular part to be contracted; and a fastening mechanism configured to contract the diameter of the tubular part by tightening the tubular part in a circumferential direction. a cylinder; a piston provided in the cylinder; a rod having one axial end side connected to the piston in the cylinder and the other axial end side extending from the cylinder toward the other axial side; a cylinder-side mounting part provided at one axial end side of the cylinder and provided to mount the rear cushion device between the vehicle main body and the rear suspension arm; and a rod-side mounting part provided at the other axial end side of the rod and provided to mount the rear cushion device between the vehicle main body and the rear suspension arm, an inner member having a cylindrical shape, fixed to the other axial end side of the rod coaxially with the rod, and having an outer peripheral surface on which a screw is formed; and an outer member mounted to an outer peripheral side of the inner member, wherein the outer member comprises: a tubular part having a tubular shape, and having an inner peripheral surface on which a screw to be screwed with the screw of the inner member is formed so that, by adjusting the screwing amount of the inner member into the tubular part, an entire length of the rear cushion device can be adjusted; and a coupling part integrally joined to the tubular part of the outer member for coupling the outer member to the vehicle main body or the rear suspension arm; a tubular part having a tubular shape, and having an inner peripheral surface on which a screw to be screwed with the screw of the inner member is formed so that, by adjusting the screwing amount of the inner member into the tubular part, an entire length of the rear cushion device can be adjusted; and a coupling part integrally joined to the tubular part of the outer member for coupling the outer member to the vehicle main body or the rear suspension arm; a slit formed in a peripheral wall of the tubular part and configured to enable a diameter of the tubular part to be contracted; and a fastening mechanism configured to contract the diameter of the tubular part by tightening the tubular part in a circumferential direction.