A piston (52) is inserted into a piston passage (53b) of a cylinder (51). The piston is displaced between a first position of a non-contact state and a second position by a pressing operation. At the first position, a suction source connection port (57) of the cylinder faces a circumferential groove (73) of the piston and a suction source conduit (33) communicates with an outside atmosphere. At the second position, the suction source connection port faces an outer peripheral surface opening (70) and the suction source conduit communicates with a suction connection port (56). A V-shaped opening portion (70a) as a correction opening portion is formed in the outer peripheral surface opening. The V-shaped opening portion causes an increment of a communication opening width at which the suction source connection port overlaps with the outer peripheral surface opening to be three times or less an increment of a pressing stroke of the piston.
