PURPOSE: To amplify an AC signal stably even when an input signal having a linear voltage change is applied to the input by connecting an inductor means in parallel with an input resistor being a component of the preamplifier circuit.

CONSTITUTION: An AC output from a sensor or the like outputted from input terminals 1, 2 connecting ground through input resistors R1, R2 respectively is amplified by a differential amplifier circuit comprising resistors R11-R17 and operational amplifiers U5, U6 and an operational amplifier U7 and outputted from output terminals 3, 4. A resistor R31 and a capacitor C1 or the like are connected in parallel with the resistor R1, it is equivalent to the connection of an inductor L1=C1×R312. An inductor is connected similarly in parallel with the resistor R2, and even when a DC component changing linearly is added to the input, the effect is reduced at the output terminals 3, 4 as time elapses, and even when a signal having a linear voltage change is added to the input signal, the preamplifier circuit amplifies the AC signal without offset stably.