PURPOSE: To prevent easily come-and-go of an output member between positive and negative operation ranges over the border position by an operational mistake of an operator.

CONSTITUTION: When a pintle 1 is advanced from a positive operational range to a negative operational range over the boundary position, the end of a liquid passage 13a is shut by switching a changeover valve 14 to the right side position, and the inside of a pressure chamber 5a is subjected to a high pressure, and thereby leftward energizing force is generated on the pintle 1, which is advanced into the negative operational range over the boundary position.