PURPOSE: To stabilize a piston operating with hydraulic pressure between two cylinders, by installing a valve in a part, interconnecting each working medium space of separate two cylinders of a hydraulic transmission type Stirling engine, and when a pressure differential occurs between each cylinder, opening this valve automatically.

CONSTITUTION: Two cylinders of a hydraulic transmission type Stirling engine prodiced with a heating part 11, a regenerator 13 and a cooler 12 are installed, while hydraulic pressure in each cylinder is led into each chamber at both sides of a piston 4, and this piston 4 is set in motion with working medium pressure variations between both these cylinders, thus engine output is secured. Working medium chambers of each of these cylinders are connected with each other at a part of a valve 8 via a pipe 5 having throttles 6 and 7. When a working medium space 21 becomes higher in pressure than the other side, a diaphragm 80 pushes a rack 82, rotating a pinion 84, whereby the valve 8 is opened, while both these working chamber spaces are interconnected so that they come to the same pressure, thereby restored to the original position.