PURPOSE: To obtain driving force for a piston by giving high and low temperatures alternately to a material capable of absorbing and emitting gas according to variation in temperatures.

CONSTITUTION: A heat exchanger 10 is provided around the external periphery of a cylinder 5 and the heat exchanger 10 stores a material 30 which emits gas at high temperatures and absorbs it at low temperatures. When gas is emitted, a piston 6 is lowered since the cylinder 5 is connected to the heat exchanger 10 and, when it is absorbed, it is raised. The up and down movement of the piston 6 is transmitted to the cylinder 5 as a rotational force through a transmission mechanism 7 including a crankshaft 21, etc. Rotation of the cylinder 5 is taken out to an external device through an output means 8 including an output shaft 28.