PURPOSE: To curtail sliding places, shorten a sliding time, and reduce drive loss of a piston.

CONSTITUTION: A pressing body 12 movable in the direction of the axis and rotationally unmovable around the axis, and a feed body 13 unmovable in the direction of the axis and rotationlly movable around the axis are arranged on the outer circumferential side of a sheath bar 7, the tubular body 6 of a piston 5, the sheath bar 7, the pressing body 12, and the feed body 13 are respectively provided with a first - a fourth ratchet teeth 21, 22, 23, 24, and the second and third ratchet teeth 22, 23 are pressed against the fourth ratchet teeth 24 by means of spring members 10, 14.