PURPOSE: To eliminate the need for an elastic packing, by defining a labyrinth seal groove having a C-shaped cross-section in a ring state on the inner circumference of a cylindrical cylinder and the outer circumference of a piston.

CONSTITUTION: A labyrinth seal grooves 20 having a C-shaped cross-section is defined in a ring state at a portion near to a lower portion of the inner surface 15a of a cylindrical cylinder 15. When a piston 14 moves upwardly, fluid in the cylinder is compressed and discharged and at the same time the pressure of the fluid causes the fluid to enter into the labyrinth seal groove 20 through a very narrow gap S. When the fluid flows against a direction in which the piston operates (in a direction shown by an arrow of a solid line), its pressure is greatly reduced by its viscosity and the pressure is further reduced when the fluid disperses and expands in the groove 20 so that the fluid is prevented from leaking. When the piston 14 is in a suction stroke (in a direction shown by a dotted line), the pressure in the cylinder 15 is made negative to sucking the fluid into the cylinder 15 so that leakage is securely prevented.