PURPOSE: To eliminate a setting error from a set speed signal of a prime mover, and to execute an absorbing operation even by a very low speed rotation by setting a value obtained by subtracting a difference of a digital speed value and a digital speed detection value from this digital speed value, as a set speed.

CONSTITUTION: Pulse pickups (pp) 6-1-6-3 detect a rotating speed of a DC electric dynamometer 2. A pulse code converting circuit 9 converts the number of output pulses per unit time of the pp 6-1, that is, this detection speed to a binary coded decimal code. A digital speed setting device 11 sets a digital set speed. A digital speed control part 10 executes an operation of [the set speed-(the set speed-a detected rotating speed)] by this digital set speed and the digital coded detected rotating speed. A speed setting control amplifier 12 converts and amplifies this arithmetic output digital value to an analog value, and obtains a set speed in negative feedback control.