PURPOSE: To make the passages of an operating gas substantially equal and improve the performance of a gas engine by connecting a heat accumulator housing manifold to a heat accumulator housing, on the other side of the end of connection between an expansion cylinder side manifold assembled with a heater tube and an expansion cylinder.

CONSTITUTION: A heater tube 4' which connects manifold 7 on a expansion cylinder 6 side to a manifold 9 on a heat accumulator housing 8 side, is extended nearly parallel to the axial line of a hot gas engine, from the both manifolds 7, 9 and formed in one pipe with its both ends connected to each other. With the positions of the expansion cylinder 6 and the heat accumulator housing 8 shifted from each other by an angle of 45°, the manifold 7 is connected to the expansion cylinder 6 and the manifold 9 is connected to the heat accumulator housing 8, placing the both connected parts in the opposite position to each other. Thereby fluid passages providing connections between the cylinder 6, manifold 7, heater tube 4', manifold 9, and housing 8 can be made substantially equal.