PURPOSE: To enable highly accurate detection of abnormality at an initial stage, by checking the abnormality of an internal combustion engine with an acoustic emission (AE).

CONSTITUTION: The detection signal from an AE sensor 20 is inputted into a main amplifier 22 through a preamplifier 21 and then, into a controller 23 vai the amplifier 22. The detection signal from an angle sensor 24 which detects the angle of rotation of a crank shaft 12 is inputted into the controller 23 to judge the beginning and the end of the intake, compression, explosion and exhaust strokes based on the signal. On the other hand, the detection signal from the AE sensor 20 is inputted into a gate circuit of the unit 23 through amplifiers 21 and 22. Then, signals of the explosion and exhaust strokes with much noise are removed by the detection signal from the angle sensor 24 inputted into the circuit and only in the intake and compression strokes the detection signal from the AE sensor 20 is inputted into an AE processing circuit to connected to a display 25 and an alarm 26.