An improved gas feed system for a gas fired diesel engine (70) is disclosed, in which a boil-off gas evaporated from a liquefied gas is fed to the diesel engine. The improvements reside in that between the diesel engine and a liquefied gas tank (11) for storing a gaseous fuel is provided a liquefied gas adding apparatus (20) for adding the liquefied gas into the boil-off gas, and that a passageway of a high-pressure gas (30) pressurized by a high-pressure compressing apparatus which compresses the boil-off gas or a mixture of the boil-off gas (50) and the liquefied gas and a passageway of a low-pressure gas pressurized by a low-pressure compressing apparatus are respectively provided in the gas feed system.
