To prevent occurrence of detect errors because of various external causes as much as possible in a sensor which detects a gap between a piston and a cylinder liner during operation of an engine.

A sensor output signal of a sensor 8a which detects a gap between a piston and a cylinder liner is inputted to a first and a second gate process circuits 21 and 22. A minimum value at the operation when a piston skirt can be regarded as being in a slide contact with the cylinder liner is taken at the second gate process circuit 22 and outputted to a zero point correction process circuit 26. The zero point correction process circuit 26 obtains a deviation of the minimum value at the operation from a reference value and corrects a zero point by adding the deviation to the sensor output. Even when the sensor is displaced by a thermal expansion difference due to a difference in material of the cylinder liner and the gap sensor, the zero point is corrected and therefore, occurrence of an error to the detect value during the operation of an engine can be prevented.