PURPOSE: To easily and speedily adjust the end of a stroke position by providing a toothed part meshing with a pinion of a shaft connected with a valve member and an end of a stroke contact member adjusting a piston stroke from the outside in a piston having an empty barrel movable in the opposite direction.

CONSTITUTION: An empty barrel 3 movable in the opposite direction is provided, and pistons 4, 5 are provided in a first separation chamber 6 and a second separation chamber 7. Toothed parts 20, 21 are provided in connection side parts of the pistons 4, 5 to mesh them with a toothed pinion 22 of a shaft 23. A first core bar 30 which abuts to the piston 4 and can be adjusted from the outside is provided in a head part of the separation chamber 6, a second core bar 36 is mounted in a head part of the separation chamber 7 so as to adjust it from the outside and a cap bush 35 engageable with the head part 38 is provided in the piston 5 to adjust a stroke on the opposite side to the first core bar 30. Consequently, it is possible to adjust a stroke end position easily and speedily, ensure the operation, and reduce cost.