A free piston Vuillermier machine which is an external combustion heat machine using a regenerative cycle, wherein a low-temperature displacer is made hollow to form a low-temperature displacer gas spring chamber (6), a low-temperature displacer rod (11) is made hollow to form a hollow chamber for accommodating a high-temperature displacer rod therein as well as a high-temperature displacer gas spring chamber (5). Further, in the above configuration, a gas spring chamber passage may be formed to make the high-temperature displacer gas spring chamber communicate with the low-temperature displacer gas spring chamber. A high-temperature displacer mechanical spring may be provided in the high-temperature displacer gas spring chamber (5), and a low-temperature displacer mechanical spring mounted on the low-temperature displacer rod may be provided in the low-temperature displacer gas spring chamber.
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