To provide a rotary inertia force assisting member and a drive wheel of bicycle equipped with the rotary inertia force assisting member, capable of indirectly assisting rotary inertia force of a gear crank, and thereby complementing a deficiency of the rotary inertia force of the gear crank caused by introduction of a free wheel.

The rotary inertia force assisting member 1 is disposed so as to be protruded on the peripheral surface of the free wheel 15 in a driving mechanism of the drive wheel 10 at least having the free wheel 15, the gear crank 20 and an endless chain 30, is rotated in the driving direction together with the free wheel 15 by rotary force in the driving direction, and increases the rotary inertia force in the drive direction of the free wheel 15. Thereby, the rotary inertia force assisting member 1 indirectly assists the rotary inertia force of the gear crank 20 connected with the free wheel 15 via the chain 30.