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