PURPOSE: To form a gas compressing system or gas engine for cogeneration compactly without leaking gas.

CONSTITUTION: In order to purify exhaust gas of a four-cycle spark ignition system gas engine for cogeneration, when a sub chamber type lean-burn system is adopted, a compressor becomes necessary. Then, an air-cooled oilless scroll compressor 4 is housed and sealed in a buffer tank together with a sirocco fan 17 or an electric motor 7, and reserve tank 6 and an after-cooler 5 are arranged outside of the buffer tank, so that a gas compressing system can be formed compactly without leaking gas.