To provide a gas energy conversion device capable of efficiently converting the energy of exhaust gas to the other medium.

A high-pressure exhaust gas is led into the pneumatic chamber 13 of a pneumatic cylinder part 10. The first to sixth hydraulic pistons 22a to 22f of the hydraulic cylinder part 20 are pressed in according to the extraction of a pneumatic piston 12 by the pressure energy of the exhaust gas. A pressure oil in a hydraulic chamber 23 is accumulated as the pressure energy in a hydraulic accumulator 44. The exhaust gas in the pneumatic chamber 13 is discharged and fresh gas is led into the pneumatic chamber 13. The pneumatic piston 12 is pressed according to the extraction of the first to sixth hydraulic pistons 22a to 22f by the pressure energy accumulated in the hydraulic accumulator 44. Then, the fresh gas in the pneumatic chamber 13 is compressed and pressurized and stored in a fresh gas accumulator 37.