1. An internal combustion engine air-fuel ratio controller, comprising: an oxygen sensor that is mounted in an exhaust path of an internal combustion engine and positioned downstream of a catalyst; a feedback control unit for exercising feedback control over the air-fuel ratio by using an output signal of the oxygen sensor so that the output signal of the oxygen sensor coincides with a predetermined reference signal; a learning unit for learning a steady component contained in a feedback control signal for the feedback control as a feedback learning value; a first integration value calculation unit for performing time integration on the feedback control signal during the time interval between the instant at which the deviation between the reference signal and the output signal of the oxygen sensor reverses from a negative value to a positive value and the instant at which the deviation reverses back to the negative value; a second integration value calculation unit for performing time integration on the feedback control signal during the time interval between the instant at which the deviation between the reference signal and the output signal of the oxygen sensor reverses from a positive value to a negative value and the instant at which the deviation reverses back to the positive value; and a learning completion judgment unit for calculating the deviation between the absolute value of a first integration value, which is calculated by the first integration value calculation unit, and the absolute value of a second integration value, which is calculated by the second integration value calculation unit, and judging, when the calculated deviation is smaller than a predetermined threshold value, that the feedback learning value is completely learned.