PURPOSE: To miniaturize the actuator for a knockout operation and to improve the durability of a valve closing spring by forming a backpressure piston diameter smaller than a valve seat diameter and opposing the output part of the actuator for knockout operation pressure drivably on valve opening side to a backpressure piston.

CONSTITUTION: The inner pressure of a plunger chamber 3a is increased up to the hydraulic pressure for knockout via an air and oil pressure booster. The hydraulic pressure acts on a backpressure cylinder chamber as well and a valve body 30 is pressed to a valve seat 23 by the valve closing force adding the spring force of a valve closing spring 31 to the differential pressure corresponding to the differential area of a valve seat diameter d0 and backpressure piston diameter d1. The valve body 30 is slightly opened by pressing it to a valve opening side by an output part 54 by driving the actuator 28 for knockout operation. The press oil for knockout is flowed out to a press oil inlet and outlet port 25. In this case the pressure opening the valve body 30 does with a small force only adding the spring force of a valve opening spring 31 to the differential pressure corresponding to the differential area of the valve seat diameter d0 and backpressure piston diameter d1. The actuator 28 for knockout operation thus does of a small output and is miniaturized.