PURPOSE: To perform quickening and smoothing action of a poppet valve without performing work of applying small drilling a hole relating to the poppet valve itself and a housing main unit, by making an output chamber always communicate with an exhaust passage through a bleed opening between a lower end small diametric part of a piston valve and a cylinder wall of the output chamber.

CONSTITUTION: A circular arc surface 21, brought into slide contact in one point at least relating to a cylinder wall 6a, is protrusively provided in the periphery of a lower end small diametric part 10A of a piston valve 10, to provide a bleed opening 22, always permitting circulating fine air between an output chamber 6B and an exhaust chamber 6A, between this circular arc surface 21 and the cylinder wall 6a. Since the bleed opening 22, which can ensure a calculated passage area, is formed between the circular arc surface 21 and the cylinder wall 6a, even when provided such as sliding the piston valve 10 temporarily, while fixedly holding a bleed flow amount of air from the output chamber 6B to the exhaust chamber 6A, this circular arc surface 21 can be brought into slide contact with low friction in one point relating to the cylinder wall 6a.