A transmission device that is adapted to engage a friction clutch that is disposed between by a drive shaft and a driven shaft, which are disposed on a first axis, by subjecting the friction clutch to a force oriented in the direction of the first axis, the transmission device comprising: a shaft member that has a male thread and is adapted to rotate around a second axis by an electric motor, the second axis being perpendicular to the first axis, the male thread being formed on the outer circumferential surface of the shaft member and centered on the second axis; a screw member that has a female thread meshing with the male thread; an arm that is adapted to reciprocate together with the screw member; a first cam member that is adapted to couple to the arm to regulate the rotation of the screw member around the second axis, and is adapted to rotate around the first axis; a second cam member that has a cam surface oriented in the direction of the first axis to oppose a cam surface of the first cam member; and a rolling member that is disposed between the two cam surfaces (21,23) to roll on the two cam surfaces (21,23); wherein the two cam surfaces are positioned on the same circumference centered on the first axis with a predetermined phase difference, wherein the transmission device is configured such that either one of the first and second cam members (20,22) subjects the friction clutch to a force oriented in the direction of the first axis to engage the friction clutch, wherein the screw member includes first sections, wherein the arm includes second sections that are adapted to come into contact with the first sections at a contact position to regulate the rotation of the screw member, wherein the transmission device is configured such that, when the shaft member makes a rotary motion in order to engage or disengage the friction clutch, the respective contact position between the second sections and the first sections is adapted to pass on a virtual plane that passes through the contact positions and the first axis, wherein a first gap that the respective contact position passes through is disposed between the screw member and the arm, wherein the transmission device is configured such that the angle formed between the second axis and the virtual plane when the friction clutch is completely engaged is closer to 90° than the angle formed between the second axis and the virtual plane when the friction clutch is disengaged. a shaft member that has a male thread and is adapted to rotate around a second axis by an electric motor, the second axis being perpendicular to the first axis, the male thread being formed on the outer circumferential surface of the shaft member and centered on the second axis; a screw member that has a female thread meshing with the male thread; an arm that is adapted to reciprocate together with the screw member; a first cam member that is adapted to couple to the arm to regulate the rotation of the screw member around the second axis, and is adapted to rotate around the first axis; a second cam member that has a cam surface oriented in the direction of the first axis to oppose a cam surface of the first cam member; and a rolling member that is disposed between the two cam surfaces (21,23) to roll on the two cam surfaces (21,23); wherein the two cam surfaces are positioned on the same circumference centered on the first axis with a predetermined phase difference, wherein the transmission device is configured such that either one of the first and second cam members (20,22) subjects the friction clutch to a force oriented in the direction of the first axis to engage the friction clutch, wherein the screw member includes first sections, wherein the arm includes second sections that are adapted to come into contact with the first sections at a contact position to regulate the rotation of the screw member, wherein the transmission device is configured such that, when the shaft member makes a rotary motion in order to engage or disengage the friction clutch, the respective contact position between the second sections and the first sections is adapted to pass on a virtual plane that passes through the contact positions and the first axis, wherein a first gap that the respective contact position passes through is disposed between the screw member and the arm, wherein the transmission device is configured such that the angle formed between the second axis and the virtual plane when the friction clutch is completely engaged is closer to 90° than the angle formed between the second axis and the virtual plane when the friction clutch is disengaged.