PURPOSE: To enhance the cooling efficiency of working gas and as well to simplify the structure of a cooler, by providing the cooler having no communication with a working gas space and including a hollow section which receives therein cooling water, integrally with the top section of a regenerator in a heat gas engine.

CONSTITUTION: A power piston 19 and a displacer piston 20 are reciprocated with a certain phase angle therebetween within a cylinder 1 which is formed therein with an opening 29 communicated with a regenerator 12. Further, several recesses 7 in a heater 6 are communicated with an opening section 8 in the cylinder 1 and an opening 19 in the regenerator 12. A cooler 13 having therein a hollow section 32 is provided on the top of the regenerator 12. With this arrangement during operation of the engine, when a high temperature working gas flows into a compression chamber 31 through regenerator gas passages 15, 17, the gas may be effeciently cooled by cooling water within the hollow section 32 in the cooler 13. It is not necessary to arrange the cooler 13 around the cylinder 1, thereby water seal and gas seal may be simply made.