When an internal combustion engine (1) is determined to be in a low-temperature start state (an affirmative determination is made in S120), an air-fuel ratio control apparatus (10) controls the temperature of a downstream detection element (17) of a downstream gas sensor (15) to a downstream target temperature by driving a downstream heater (16) of the downstream gas sensor (15) (S170), and feedback-controls the air-fuel ratio of exhaust gas based on the output of the downstream gas sensor (15) (S190). When the engine (1) is determined not to be in the low-temperature start state (a negative determination is made in S120), the air-fuel ratio control apparatus (10) drives an upstream heater (25) of an upstream gas sensor (22) and feedback-controls the air-fuel ratio of exhaust gas based on the output of the upstream gas sensor (22) (S270).