An amplifier circuit includes a converter configured to convert a predefined physical quantity to a resistance value, and the resistance value converted by the converter is converted to a voltage value and then amplified. The converter includes variable resistance sensors of piezoresistance elements. A bias unit is configured to determine a bias current of the converter, and includes bias resistances. An operation amplifier unit receives, as input signals, output signals from the bias unit and the converter, and includes feedback resistances respectively connected to input and output ends of a first operational amplifier. The first operational amplifier is a whole differential operational amplifier including a common-mode feedback circuit.