To provide a high-precision judgment by calculating the power of a compressor based on the detected values of the engine revolving speed and the high pressure and low pressure of the compressor, determining the throttle opening based on the power, and judging an engine misfire from the deviation between the calculated value and detected value of the throttle opening.

Compressor power is calculated by the compressor power arithmetic unit 21 of a controller 10 based on the detected values of an engine rotation detector 6, a high-pressure detector 9, and a low-pressure detector 8, and the engine output is obtained by an engine output arithmetic unit 22 from the compressor power. The throttle opening S1 corresponding to the engine output is obtained by a throttle opening arithmetic unit 23. The deviation between the actual throttle opening S2 by a throttle opening detector 7 and the throttle opening S1 is obtained by a throttle opening comparator 24. When the deviation exceeds a prescribed value, the controller 10 judges that a misfire occurs on at least one cylinder of the engine and operates a warning device 11 and an engine stopping device 12.