A first bias voltage generating circuit which applies a bias voltage to an amplifier circuit of an AD converter has a driving unit and a control unit. The driving unit includes a first bias circuit and a second bias circuit as a plurality of bias circuits which are connected in parallel and have different current driving capabilities. The first bias circuit and the second bias circuit each include a CMOS transistor which is connected directly between a power supply potential and a ground potential, and a switching element which interrupts a feedthrough current. The drains of the CMOS transistors output the bias voltage. The control unit turns on both or either one of the first bias circuit and the second bias circuit, thereby controlling the current driving capability of the entire driving unit.