PURPOSE: To promote efficiency by connecting operation chambers of four, large and small, rotary piston machines respectively via heat exchangers.

CONSTITUTION: Operation gas is conducted into the operation chamber of a large-diameter rotary piston 41b from a pipe 46, compressed and introduced into a cooler 52 by the revolutions of the piston for isothermal compression. The gas therefrom is intrduced into a combustor 54 by a small-diameter rotary piston 42b for isovolumetric heating. This gas is conducted into a combuster 53 by a small-diameter rotary piston 43b, and then introduced into a large-diameter rotary piston 44b for isothermal expansion, and an output is taken out from this piston 44b. The gas thereafter is cooled by a regenerator 54, and introduced into a large-diameter rotary piston 45b to be isovolumetrically cooled for completing a stirring cycle.