To connect an amplification circuit without usage limit of input impedance by forming a bridge of four piezo resistor whose resistance values are changed according to the detected physical quantity, connecting a pair of operation amplifiers and outputting a difference between both output voltages.

In piezo resistor R1 to R4, their resistance values are changed according to the detected acceleration quantity. In an operation amplifier OP1, its output terminal is connected with an inverse input (-) terminal connecting to a resistance Ra for setting current value, by negative feedback through a serial circuit consisting of resistor R1 and R2, and a constant voltage Vref is supplied to a non-inverse input (+) terminal. In an operation amplifier OP2, its output terminal is connected with a negative terminal by negative feedback through a resistor R3, and its positive terminal is connected with the connection point of resistor R1 and R2, then the negative terminal is connected with the other end of a resistor R4 whose one end is connected with the negative terminal of the amplifier OP1. The difference of voltage between both output terminals of the amplifiers OP1 and OP2 is outputted as sensor output. Thus the output impedance drops, thereby eliminating the usage limit of an amplifier due to input impedance.