PURPOSE: To facilitate the zero point adjustment by lighting a display element such as a light emitting diode, thereby indicating the zero point of the sensor.

CONSTITUTION: If the adjustment of a variable resistor VR which adjusts the zero point of the sensor S1 is correct, the output of a first inverter X1 becomes 1 and the output of a second inverter X2 becomes 0. The light emitting diode PD1 intermittently flashes at a period of an oscillator H, and a relay RY does not operate. Therefore, it is found that the zero adjustment has been correctly performed. When the sensor S1 senses an object to be sensed, both inputs to an exclusive OR circuit 1 become 0, the oscillation of the oscillator H is stopped, and the light emitting diode PD1 is continuously lit. At this time, the relay RY is operated, and warning is issued.