An assistive power control apparatus for a motor-assisted bicycle controls a motor to generate an assistive torque depending on the pedaling force of a rider. The assistive power control apparatus detects a rotating torque value of a crankshaft rotated when a rider steps on pedals by a torque sensor, controls a motor to generate an assistive power determined using the rotating torque value, calculates a rotation angle of the crankshaft when a present rotating torque value is detected from a time period from a timing of detection of a peak position of the rotating torque value to a timing of detection of the rotating torque value, calculates a pedaling force applied when the rider steps on the pedals from the calculated rotation angle of the crankshaft and the detected present rotating torque value, and determines the assistive power of the motor according to the calculated pedaling force.