PURPOSE: To supply a pressurized air to upper and lower pistons through a simple machining of a spindle by ventilating the pressurized air supplied to the lower side of the lower piston to the lower side of the upper piston.

CONSTITUTION: A pressurized air is introduced directly to the lower side of a lower piston 18a from an air pressure introducing hole 21 of a cylinder 20. The introduced pressurized air pushes up a lower piston 18b, enters a lower connecting hole 19b from the lower face of the lower piston, and then reaches the lower side of the upper piston 18a from an upper connecting hole 19a through upper and lower shaft-holes 35a, 35b of upper and lower spindles 17a, 17b. The pressurized air reached the lower side of the upper piston 18a pushes up the upper piston 18a. Thus, the pressurized air acts upon the lower faces of the upper and lower pistons 18a, 18b.