PURPOSE:To reduce the damage of a valve body while preventing water in service interruption from keeping flowing by housing a plunger head in an end of a plunger to be urged toward the end. CONSTITUTION:A tubular plunger 4 is mounted in an actuator casing 3. A plunger head 6 is housed in the tubular plunger 4 to be always urged toward an end thereof by a spring S, and piezoelectric elements a-d are disposed on the outer peripheral side of said plunger 4. When the tubular plunger 4 is operated in the end advancing direction, the advancing operation with a strong press force due to the action of spring S can be prevented at the final stage, so that a valve body can be prevented from damage. Also in service interruption, when the tubular plunger 4 is released from clamping, the plunger head 6 automatically advances toward the end, so that the valve body can be closed to prevent water being keeping flowing in the service interruption.
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