To provide a reciprocation type expansion compressor capable of more stabilizing reciprocating motion of a free piston with a very compact and simple structure, converting expansion work of working fluid of a gas into compression work, and acquiring power generation output.

This reciprocation-type expansion compressor 1 includes a cylinder 2, the free piston 5 reciprocatably received in the cylinder 2, and dividing the cylinder 2 into an expanding section 3 and a compressing section 4, a high-pressure suction valve VH1 and a low-pressure discharge valve VL1 communicated with the expanding section 3 and opening and closing a flow channel, and a low-pressure suction valve VL2 and a high-pressure discharge valve VH2 communicated with the compressing section 4 and opening and closing the flow channel. The expansion work of the refrigerant of high pressure in the expanding section 3 is converted into the compression work of the refrigerant of low pressure in the compressing section 4. A motor needle 11 is disposed on an outer peripheral section of the free piston 5, and a motor stator 16 is disposed at an outer peripheral side of the motor needle 11.