PURPOSE: To develop great torque from a predetermined working force without increasing any rotational locus of a crank arm.

CONSTITUTION: A crank arm 6 is formed of a first arm 7 and a second arm 8 connected together slidably in the longitudinal direction, and in the connection part between the respective arms 7, 8, a pinion 13 interlocking with the crank arm 6 for rotating is arranged. In the second arm 8, rack teeth 17, 18 facing parallelly to each other are arranged, and engaging teeth 19 arranged in the pinion 13 are meshed with the rack teeth 17, 18 alternately in every half rotation of the crank arm 6. In this structure, the overall length of the crank arm 6 is extended/shrunk in every half rotation, and great torque is generated when external force is applied in the extended position.