PURPOSE: To judge whether an opening sensor exerts the specified performance easily by comparing the section air amount determined from the revolving speed with the suction air amount determined from the output of an air flow sensor while the degree of throttle opening is held in the steady state.

CONSTITUTION: An A/N calculating part 9 connected with an air flow sensor 2 calculates the measured air amount A/Ni per revolution. This calculating part 9 is connected with a steady state judging means consisting of a measured air amount filtering part 10, deviation calculating part 11, and steady judging part 12, and if it is judged to be steady state, the output switch 13 of a failure judging part 16 is closed. Meantime the degree of opening θ of a throttle valve 3 is sensed, and the presumative suction air amount A/N is calculated 14 from the engine revolving speed, entered into a deviation calculating part 15, and compared with the measured air amount A/Ni. Thus the deviation determined is put out. When the deviation exceeds the specified valve, the failure judging part 16 judges that the opening sensor 8 is troubled, and an appropriate signal is emitted. Thus trouble-shooting can be made with simple configuration.