A controller of an air-fuel ratio sensor includes, in a digital processor, an adjuster and a control unit, among which the adjuster adjusts a second input digital value for a control of an output voltage of a second voltage application circuit to a preset voltage, and the control unit controls, based on a calculation result of a digital value of a sensor current, a first input digital value for a control of an output voltage of an output terminal of a first OP amplifier of a first voltage application circuit to a voltage Vu based on the equation, Vu=Vout±(Vtar+Is×Rs), so that an application voltage applied to the air-fuel ratio sensor is quickly adjusted to a target voltage.