To provide a regenerator of an external combustion type heat gas engine capable of inhibiting heat loss.

A regenerator 4 of this external combustion type heat gas engine is internally mounted on an operation gas circuit of an external combustion type gas heat engine and is adapted to absorb and release heat energy of an operation gas flowing reciprocatingly. This generator 4 is fitted in the outer periphery of a piston 2 of a heat gas engine in shape. A sleeve 100, which is made of a low thermal conductive raw material, is laid out on the inner peripheral area of the regenerator 4 which is opposed to the outer surface of this piston 2.