To avoid an influence by performance dispersion of each part in an assist power mechanism so that operation can be made by light stepping force without a feeling of physical disorder.

A bicycle is provided with a load sensor 21 interposed between stoppers of support frames 6, 6 of a pedal crankshaft to detect reaction force of a deceleration output shaft, speed sensor 23 counting a number of teeth of a chain sprocket 4 to detect a rotational speed and a controller 41 rotation drive controlling a motor. By a product of a detection value of a detection means 22 of tensile force of a chain 5 and a winding radius r0 of the chain sprocket 4, resultant torque is calculated, from a product of a detection value of the load sensor 21 and a distance r3 from a pedal crankshaft center of this load sensor 21, torque of the deceleration output shaft is calculated, to be compared with the resultant torque, so as to calculate torque ratio η, so as to be approximated to target ratio, an applied current to an assist motor is adjusted, motor rotation is controlled to be synchronized with rotation of the chain sprocket 4.