PURPOSE: To reduce the number of parts of a circuit by providing an A.C. machine to detect the current in a cable way, a current-voltage converter, a signal amplifier of a CMOS structure, and a standard voltage generator to apply a standard voltage for offset.

CONSTITUTION: The current in a cable way is detected by an A.C. machine 1, and the output current is converted into a signal voltage by a current-voltage converter 2. The signal voltage is input to the input terminal of a signal amplifier 3 of a CMOS structure so as to amplify. To the + input terminal of the amplifier 3, a standard voltage to be offset is applied from a standard voltage generator 4, and the superposing of the offset and the signal amplification are carried out by a single operation amplifier. And by inputting the standard voltage to one of A/D converters of a microcomputer 32, the zero level of the signal voltage can be decided easily. Since the amplifier 3 is a CMOS structure, it can be operated by a single power source, and the power source circuit can be made simple so as to reduce the number of parts.