PURPOSE: To perform stable and highly accurate speed control by changing the output period of speed change signals in simple increase according to the quantity of deviation in performing the feedback control of a speed change means according to the deviation of the actual revolution of a stepless speed change gear from the objective revolution.

CONSTITUTION: In the case where a mechanical type continuously variable transmission 1 is rotated by a motor 2 for rotation, the waveform of a output revolution pulse signal detected by a magnetic sensor 8 is input in a micro computer 12 after being reshaped by a waveform reshaping circuit 10. The number of the detected pulses included in the output cycle Tz of the timing signal Sz from a timing controlling section 18 is counted by a counter 14, and the value is given to a actual revolution calculating section 18. In this section, actual revolution Sm is calculated from the value obtained by counting. In the next step, deviation (e) from objective revolution So is determined by a deviation operation section 20. The rotation direction and conduction period Tn of a reversible motor 4 are controlled according to this deviation (e). At this moment, the conduction period Tn of speed change signal Sn is changed in simply increase according to the deviation (8).