PURPOSE: To enable a sufficient amount of cold heat for a heat exchanging operation to be produced without applying a substantial investment in facility and without increasing pressure of product synthetic gas.

CONSTITUTION: A synthetic gas refining device for manufacturing product synthetic gas S having a specific composition rate is provided with a heat exchanger 2, a gas-liquid separator 3 and an expansion valve 4 within a refrigeration box 1 to which a thermal insulation is applied, and at the same time an expansion turbine 5 is installed inside or outside the refrigerating box 1. Then, the raw synthetic gas S is fed into the gas-liquid separator 3 through the heat exchanger 2 and gas gasified within the gas-liquid separator 3 is discharged out of a system as the product synthetic gas Sa through the heat exchanger 2. Liquified gas F stored at the bottom part of the gas-liquid separator 3 is supplied to the expansion turbine 5 through the expansion valve 4 and the heat exchanger 2, off-gas O expanded in thermal insulated state at the expansion turbine 5 is discharged out of the system through the heat exchanger 2.