PURPOSE: To correct a detection voltage outputted from a physical quantity detection means automatically to a correct value by calculating a voltage fluctua tion rate automatically when a reference voltage given to the physical quantity detection means is fluctuated.

CONSTITUTION: A reference voltage VB inputted to a multiplexer 10 and a detection voltage from a physical quantity detection means 3 are selected alternately and given to a computer 6 via an A/D converter 5, the computer 6 calculates the voltage fluctuation rate of the reference voltage on each occasion and the result of calculation is utilized to give a command to correct the current detection voltage to an operational amplifier 4 or the A/D converter 5 or the said computer 6 from the computer 6. As a result, the current detected voltage is automatically corrected to a correct value at all times.