PURPOSE: To prevent a mechanical break-down by supplying current from a temperature detecting circuit through a control circuit to a control coil to avoid an excessive stroke of a piston even in high temperature of the atmosphere.

CONSTITUTION: In low temperature of the atmosphere, a transistor 21 is turned off so that a normally opened contact 22a of a relay 22 is opened. In high temperature of the atmosphere, the transistor 21 is turned on and the contact 22a of the relay 22 is closed so that direct current is supplied to a controlling coil 11 which acts as a direct current magnet. Thus an attraction acts on a controlling moving iron core 12 before and after a stroke of a piston to control the stroke in the direction of reducing the stroke. Thus, a mechanical breakdown can be prevented.