To provide a pedaling force detecting device of a motor-assisted bicycle.

At least one pair of helical gears are mounted in the transmission system of the motor-assisted bicycle. The helical gears are driven with the pedaling force to be rotated locally, and another helical gear is engaged and driven with the corresponding helical gear on the shaft of the helical gear to make horizontal movement. An elastic body is attached to the end side of the helical gear, and annular magnets moving according to the helical gear are attached to the elastic body and the end of the shaft of the helical gear. A hole detector is attached to the stationary fixed end opposite to the moving end side of the annular magnet. Accordingly, based on the principle that the helical gear in the transmission system generates the sideways force, the quantity of movement of the helical gear in the axial direction is detected, and the voltage signal is outputted thereby to control the motor. Then, the power of the motor is outputted to the motor-assisted bicycle to achieve the purpose of the motor assistance.