To separately judge a degradation degree of an air quantity sensor and a fuel injector by calculating a difference between a measured air quantity and an actual suction air quantity and a difference between a fuel injection quantity command value and an actual fuel injection quantity from a difference between an air-fuel ratio detecting value and the desired value and a difference between an output shaft torque detecting value and the target value.

A control unit performs operation processing by inputting signals from an accelerator angle sensor, an air quantity sensor, a crank angle sensor, a water temperature sensor, an air-fuel ratio sensor and a torque sensor to output a pulse signal to a fuel injector and a target signal of throttle opening, and also calculates a difference ΔQa between a measured air quantity and an air quantity actually flowing in a cylinder and a difference ΔGf between a fuel injection quantity command value to the fuel injector and an actual fuel injection quantity from a difference ΔA/F between an air-fuel ratio detecting value and the desired value and a difference ΔTe between an engine output shaft torque detecting value and the desired value. Failure of the air quantity sensor and the fuel injector is diagnosed by judging whether or not these differences ΔQa, ΔGf fall within a prescribed range.