PURPOSE: To prevent increase in control force necessary for operation by selectively opening a low pressure connection and an atmospheric pressure connection by a valve body acted on by a spring, and providing excitation current of a magnetic coil as control variable for the position of a throttle valve.

CONSTITUTION: A chamber 50 communicates with an atmospheric pressure connection 24. A diaphragm 42 is moved upward by a spring 44 under high pressure. A valve body 120 is moved upward by a predetermined current. The pressure connection 24 is closed, a low pressure connection 28 is open and a chamber 52 is under low pressure. The pressure in the chamber 50 is decreased by a gap 154 and through holes 156, 152. The diaphragm is moved downward, and the valve body is in an intermediate position. When control current is increased to some extent when the valve body is in the intermediate position, the valve body 120 is elevated, however the diaphragm moves the valve body to the intermediate position again. When the control current is decreased to some extent, the valve body 120 is moved downward, however the valve body 120 is returned to the intermediate position again. Thus, as the control current not only regulates the valve body, but also the diaphragm 42, the regulation of a control valve 8 can be effected correctly.