PURPOSE: To protect a piston by providing an operating condition sensor which includes vibration of the engine concerned, installing an abnormality sensing means to sense abnormal vibration of the engine on the basis of signals given by this operating condition sensor, and by furnishing a control means to reduce the revolving speed of the engine more when the vibratory value is higher on the basis of signals given by this abnormality sensing means.

CONSTITUTION: The reference value for sampling time and the vibration setting value given by a setting device 12 are corrected by a correcting device 20 into a proper value on the basis of signals from an operating condition sensor 14, and the correction value as output signal therefrom is fed to an engine abnormality sensor 30. Output signal from a vibration sensor 28 of the operating condition sensor 14 is fed to this engine abnormality sensor 30 upon being amplified by an amplifier 32. The correction value from the correcting device 20 is compared at this engine abnormality sensor 30 with signal from the vibration sensor 28, and any abnormal condition of engine is sensed, and the output signal therefrom is fed to an abnormality handling signal emitting device 34. The output signal therefrom actuates an alarm device 36, and an engine deceleration/stop device 40 is operated to decelerate the engine. When the vibration level is high, the engine shall be either decelerated quickly or stopped.