To prevent a motor assist ratio of a transmission from changing due to shifting operation by interposing the transmission capable of changing its speed reduction ratio in a human power drive system and controlling power or time rate of work of a motor drive system according to changes in the power of the human power drive system using a predetermined conversion formula or a conversion map.

Continuous torque T estimated with interpolation means 80 and a crankshaft rotation speed N obtained from speed calculation means 78 are added up with an adder 81. The result is inputted to motor power target value calculation means 84. The means 84 has a target value stored corresponding to an actual value of human power in the form of a map or a formula and determines a target value of a drive force or a power of a motor 64 using a conversion map or a conversion formula, compares the target value with an actual value 100 of the motor power comparator 86, and controls the power of the motor 64 so that the comparative difference becomes zero. Thus, motor assist ratio of a transmission is prevented from changing due to shifting operation.