To provide an abnormality deciding method capable of specifying a fuel force feed system generating an abnormality without fluctuating fuel pressure within an accumulator piping.

A fuel pump 30 is provided with a first supply pump 50a and a second supply pump 50b. Fuel is alternately and forcibly fed from each supply pump 50a, 50b to a common rail 20. Each injector 12 executes a fuel injection based on fuel pressure (rail pressure) within a common rail 20. An electronic control device (ECU) 60 detects rail pressure rising amount for a fuel force feed period and calculates an estimated value of the rail pressure rising amount based on a force feed command value for a fuel pump 30. The ECU 60 judges whether the fuel force feed period is a period when either one fuel force feed of each supply pump 50a, 50b is performed. An abnormality of each supply pump 50a, 50b is separately decided based on the detected value and the estimated value of rail pressure rising amount.