PURPOSE: To simplify the detection of faults in an amplifier circuit system, by inputting the output of a bias circuit into an operation amplifier through a switch, in a circuit wherein the output of a load cell is inputted into an A/D converter through the operation amplifier.

CONSTITUTION: The output ends of a bridge 1 of a load cell are connected to positive and negative input terminals 2a and 2b of an operation amplifier 2. A feedback resistor Rf is connected between an output terminal 2c and the input terminal 2b. The output terminal 2c is further connected to an A/D converter 4 through a low pass filter 3. One end of a bias circuit D is connected to an applied voltage input terminal. The other end of the bias circuit D is connected to the input terminal 2b through a switch SW. When the switch is closed, the amplifier is functioned as an adder. At first, the switch SW is closed and the circuit D is adjusted so that the display value becomes a specified weight. Then the switch is opened and measurement is performed. Thus with the switch SW closed, the display value is checked and the fault in the amplifier circuit system can be can be detected.