PURPOSE: To lessen shock to a prime mover when it is operated at high speeds as well as to improve suspension performance when it is operated at low speeds.

CONSTITUTION: Since speed command signals are so designed as to pass through a first time constant circuit 4 high in time constant by concurrently switching a second analogue switch 5 to a terminal 5a side with a first analogue switch 3 switched over to a terminal 3a side when a prime mover is operated at high speeds, the prime mover can moderately follows change in speed command signal, so that shock to which the prime mover is subjected at the time of rotation and suspension can thereby be relaxed. And since the speed command signals are also so designed as to pass through a second time constant circuit 8 low in time constant by concurrently switching the second analogue switch to a terminal 5b with the first analogue switch 3 switched over to the terminal 3b side when the prime mover is operated at low speeds, the prime mover can thereby quickly follow change in speed command signal, so that excellent suspension performance can be obtained.