A piston is inserted into a piston passage of a cylinder. 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 of the cylinder faces a circumferential groove of the piston and a suction source conduit communicates with an outside atmosphere. At the second position, the suction source connection port faces an outer peripheral surface opening and the suction source conduit communicates with a suction connection port. A V-shaped opening portion 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.
