PURPOSE: To calculate a stable power source voltage value by deriving a difference between an output value of a first arithmetic circuit which has contained an A/D converted value of a power supply voltage as an element of an arithmetic operation, and a prescribed value corresponding to a reference power supply voltage, by a second arithmetic circuit, and storing this adjusted value in a storing circuit.

CONSTITUTION: When a power supply voltage Vcc is set to a reference voltage BTr by a reference voltage circuit 7, an output of this reference voltage BTr is A/D converted, and thereafter, goes to a digital value of BTx, and this BTx is a product which is subtracted an output error α of an A/D converting circuit 2 from the reference voltage BTr. Also, a difference between an output of the A/D converting circuit 2 and a data BTr which is set to a reference voltage fixing data circuit 3 is derived as an adjusted value DBT by a subtracting circuit 4, and at the time of adjusting a power supply voltage calculated value, when an adjustment mode switch 9 is turned on, the adjusted value DBT, namely, the error α is stored in an adjusted value storing circuit 5. When this stored adjusted value and the output value of the A/D converting circuit 2 are calculated by an adding circuit 6, an error by the A/D converter 2, etc., is negated, and a stable power supply voltage calculated value containing no error is obtained from the adding circuit 6.