To reduce the cost and the space for an analog input circuit by simplifying the constitution of a gain switching amplifier circuit amplifying plural kinds of analog input signals at different levels.

An analog signal is inputted to the positive-phase input terminal (+) of the operating amplifier 31 of a positive amplifier 3. Resistors 35 and 36 are connected to the negative phase input terminal (-) of the operating amplifier 31 and grounded. One transistor 5 is provided. The collector of the transistor 5 is connected to the connecting point of the resistors 35 and 36 to ground an emitter. The kind of an analog input signal is detected from an amplifying voltage level by an A/D converter 2 to turn on/off the transistor 5 by means of CPU 4 corresponding to the kind of the analog input signal to switch the gain of the amplifier 3. A voltage VBC between a base and the collector when the transistor 5 is off is set VBC0.6V to make a current between the base and the collector nearly zero to make the output of the amplifier 3 substantially no distortion.