To provide an engine suited for an air conditioner, refrigerator, or the like by forming the total unit into small size of a free piston type Vuilleumier cycle engine.

An external combustion engine is formed in a constitution using a regenerative cycle provided with a gas passage 107 connecting high/low temperature part intermediate temperature chambers 102, 103. Here, a low temperature displacer rod 111 is formed hollow as a hollow chamber storing a high temperature displacer rod 110, the inside of a low temperature displacer 109 is made hollow as a low temperature displacer gas spring chamber 106, the hollow chamber is protruded in this spring chamber 106 to serve as a high temperature displacer use gas spring chamber 105, and a gas spring chamber passage 120 for communication between the spring chamber 105 and the spring chamber 106 is formed. A high temperature displacer use mechanical spring 118 is built in the inside of the high temperature displacer gas spring chamber 105, and a low temperature displacer use mechanical spring 119 mounted in the low temperature displacer rod 111 is desired to be provided in the low temperature displacer use gas spring chamber 106.