PURPOSE: To prevent defective operation of a change-over mechanism due to heat thereof and improve the injection property and combustion property of fuel by providing a check valve having high heat resisting property and pressure resisting property in the change-over mechanism for a pressurized air supply source.

CONSTITUTION: While a fuel injector Z1 supplies pressurized air from an air tank 3 to a fuel injection valve 2 in the start of an engine having low discharge pressure of a compressor 1, it shuts off air from the tank 3 to supply pressurized air from the compressor 1 when the discharge pressure reaches at least a set value. A check valve 15 having high heat and pressure resisting properties is arranged at the very hot air path 21 side of this change-over mechanism so that a solenoid change-over valve 6 and a constant differential pressure valve 7 in an air path 22 connected from the air tank 3 to the downstream air side of the check valve 15 are not exposed directly to hot air from the compressor 1, the solenoid change-over valve 6 does not have any defective operations due to temperature rise and the change-over operation of an air supply source has no troubles.