PURPOSE: To ensure a high-accuracy detection of the voltage deviation with a high reliability and no malfunction caused by the thermal effect or the like, by using the constant current generating circuit, the voltage generating circuit, the reference voltage setting circuit and the comparator circuit each.

CONSTITUTION: The constant current generating circuit 4 which generates constant current is connected to the stabilizing power surce 2. The voltage generating circuit 12 receives a supply of the constant current from the circuit 4 and then converts the displacement applied from outside into the voltage variation in relation to the constant current output to generate the voltage output. The reference voltage setting circuit 20 provided in correspondence to the circuit 12 sets both the upper and lower limit reference voltage VH and VL to decide the range of the reference voltage deviation. These VH and VL are supplied to the comparator circuit 30 in the form of the comparison voltage and together with the output voltage V of the circuit 12. The circuit 30 compares the output voltage V of the circuit 12 with the voltage VH or VL, and the output is produced when the deviation of the voltage V gets out of the range of the reference voltage deviation.