An air-fuel ratio sensor is equipped with a sensing element including a solid electrolytic substrate. When the sending element is warmed up and activated by a heater, a microcomputer controls electric power supplied to the heater based on a control base value being set according to a duty ratio=100%. A power profile P1 is determined beforehand to set a target heater power. Through the warm-up heater power control, an actual heater power supplied to the heater is equalized to the target heater power determined according to the power profile P1.