The present invention relates to control device for electric bicycle, and is especially output power controlling device for electric bicycle. The output power controlling device includes two Hall sensors and magnets moving with the bicycle, and features that these two Hall sensors have the S poles connected to the power source, the D poles earthed, one G pole connected serially with one voltage dividing reactance device before connected to one other G pole in parallel and to the output, and one other reactance device connected between the output and the power source. The output power controlling device has fast reaction speed, high accuracy and easy use in electric bicycle.