A downstream exhaust system controller (10) generates a target value for the output of an upstream O 2 sensor (7) disposed between first (3) and second (4) catalytic converters for converging the output of a downstream O 2 sensor (6) disposed downstream of the second catalytic converter to a target value while taking into account the dead time of a downstream exhaust system (E1). An upstream exhaust system controller (11) generates a target air-fuel ratio for an internal combustion engine (1) for converging the output of the upstream O 2 sensor to a target value while taking into account the dead time of an upstream exhaust system (E2). A fuel processing controller (12) controls the air-fuel ratio of the internal combustion engine at the target air-fuel ratio according to a feedback control process.