A tightly closed casing has an inside space for storing coolant gas. A
block is accommodated in this casing. A motor includes a stator and a
mover. A piston is connected to the mover of the motor. A movable element
includes the mover of the motor and the piston. A stationary element
includes the stator of the motor and the block. An elastic element has a
portion fixed to the movable element and another portion fixed to the
stationary element. A cylinder is shiftable in an axial direction with
respect the block. A shifting device shifts the cylinder in the axial
direction.
