To attain highly precise monitoring or diagnosis for previous conveyance pressure in the fuel distribution pipe of a fuel feed system by detecting the previous conveyance pressure with a pressure sensor mounted on the fuel distribution pipe, in operating an internal combustion engine (Otto engine) in an operating mode where a pressure rise to high pressure is made inactive.

A fuel feed system in an internal combustion engine forcibly feeds fuel in a fuel tank 1 with a motor-driven fuel pump 2 having a pressure controller, and feeds the fuel whose pressure has been raised, to respective fuel injection valves 7 from a fuel distribution pipe (rail) equipped with a pressure sensor 5. The rail 4 is connected to the suction side of a high-pressure pump 3 with a control circuit 8 having a pressure control valve 6, so that continuous fuel flows are generated. In diagnosing a conveyance fuel system, an internal combustion engine is operated with the pressure control valve 6 opened, the previous conveyance pressure is detected by the pressure sensor 5 one after another. The self-diagnosis of the fuel injection system is performed based on the detected results.