PURPOSE: To perform smooth switching between automatic operation and foot operation, by controlling the displacement of a clutch plate in accordance with an engine speed.

CONSTITUTION: An externally equipped resistor 13R of a one-shot multivibrator 13 in an engine speed detection circuit 1 is adjusted to set an engine speed corresponding to a lower limit output Vmin of an amplitude limiting circuit 2 to, for instance, 1,000rpm; if the engine reaches this speed, operation of a clutch is started. Here a clutch plate is advanced, though the engine speed for the maximum displacement of the clutch plate is specified by an upper limit output Vmax of the circuit 2, if the corresponding speed is set to 3,000rpm, the clutch is 100% connected when the engine reaches said speed and the connection is completed. Then if a clutch release switch 91 is operated, the clutch is placed under an off condition.