Immediately after startup of the engine, secondary air is supplied by an air pump (ll) into each exhaust branch passage (8) via a secondary-air supply passage (13). A pressure sensor (36) is disposed in the secondary air supply passage (13). An output voltage of the pressure sensor (36) decreases as the battery voltage decreases. When the battery voltage is lower than a permissible voltage, the output value of the pressure sensor (36) is inhibited from being used as an output value representing the pressure. Thus, atmospheric pressure can be accurately detected.