To provide a Stirling engine that reduces energy loss and has a high refrigerating capacity from a low amplitude to a high amplitude without deteriorating a piston spring by varying a gas quantity returned to a compression chamber from outside a cylinder depending on an engine operating state.

The Stirling engine, which is a free piston type Stirling engine having a displacer 1 and a piston 2 disposed for reciprocation in the cylinder 9 filled with working gas, comprises flow passages 13 and 14 for interconnecting a compression space 8 between the displacer 1 and the piston 2 and an external space 12 about the cylinder 9, and an oscillation center fluctuation preventing means, such as a needle valve 20, disposed separately from the flow passages 13 and 14.