To enable power assisting while using the most of the convenience in use in a conventional bicycle.

This bicycle judges whether the rotational frequency of a crank becomes a proper value or not to actual travelling speed V1 according to whether rising of a rotation signal of a sensor unit is detected or not during a permissible value (t) is passing (s13, s16). When the rising of the rotation signal is detected, the crank is judged to be suitably rotated, and power assist is carried out by a motor (s17, s18, s25). When the rising of the rotation signal is not detected, the rotation of the crank is judged to be deficient, and power assist by the motor is actually not carried out (s22, s25). In this way, power assist is carried out with exacting crank rotation by a driver, thereby ensuring drive feeling similar to a conventional bicycle. By exacting power assist proportional to the operating amount of a throttle grip on the precondition that the crank is suitably 1 rotated detailed driving control by a driver's will can be accomplished (s17).