a pressure container filled with a working gas;

a cylinder secured inside the pressure container;

a power piston provided inside the cylinder; and

a displacer provided inside the cylinder on a same axis as the power piston and elastically supported with a supporting spring,

wherein the pressure container comprises: a work space located on a displacer piston side of the power piston; and a back-pressure space located on a side of the power piston opposite to the work space,

wherein the displacer comprises: a displacer piston that slides inside the cylinder; and a rod which is connected and fixed to the displacer piston and placed through a slide hole formed at a center of the power piston,

wherein the displacer piston has a hollow space inside,

wherein the displacer piston has formed therein: one or more than one inlet via which the working gas flows into the hollow space inside the piston; and one or more than one outlet via which the gas having flowed into the hollow space flows out of the hollow space,

wherein the inlet is formed in a wall surface to which the rod is connected, the inlet penetrating the wall surface from outside the wall surface into the hollow space,

wherein the outlet is formed in a side circumferential wall of the displacer piston, the outlet penetrating the side circumferential wall from the hollow space to outside an outer circumferential surface of the displacer piston, and

wherein the rod is formed in a shape of a hollow pipe, and is fitted with, at one end thereof, a member for minimizing flow of the working gas between the back-pressure space and the hollow space.