PURPOSE: To permit stable starting which will never be affected by a load condition, by a method wherein a gap between the fixed electrode and a movable electrode of a variable resistor, into which electroconductive liquid is encapsulated by an insulated vessel, is reduced in accordance with the counter electromotive force of a motor.

CONSTITUTION: When a switch 2 is closed, a predetermined voltage E is impressed on a motor M. In this case, a solenoid coil 15 is excited by a counter electromotive force Ea, generated in the armature of the motor, and pushes a movable core 13 down toward a fixed electrode 9 against a coil spring 14. Accordingly, a gap G between the fixed electrode 9 and a movable electrode 10 is reduced, whereby the value of resistance Rs of a variable resistor 5 is reduced linearly. When the gap G has arrived at a minimum gap point, a microswitch 19 is operated and a short-circuit contactor 3 is closed, whereby the motor M is accelerated to the maximum speed thereof.