To provide the fuel supply system of an internal combustion engine capable of improving the overheat preventing performance of the engine.

When an engine rotational frequency NE exceeds an engine rotational frequency NEOH (step S4) and an engine water temperature TW exceeds an engine water temperature TWHO (step S5) and a vehicle speed VP is below a vehicle speed WFC1 (step S6), by means of a delay of a specified time set by means of a delay timer tmOH (step S7), the engine rotational frequency NHFC1H (/L) is reduced by a specified reduction quantity DNFCV. Thereafter, reduction is carried out by a reduction quantity DNFCV per specific period of time (step 2) set by a timer tmTDC, and as the result, the engine rotational frequency NHFC1H (/L) is gradually reduced. Next, a flag FNHFC which shows gradual reduction of the engine rotational frequency NHFC1H (/L) by [1] is set to 1 (step 9) and when this flag FNHFC is set to 1, an air conditioner clutch is set OFF.