To ensure a stable combustion state never influenced by change in temperature and humidity of the outside air by season, place or time.

According to this intake air introduction method for engine, stable intake air supply and a stable combustion state can be ensured by filtering, as intake air for an engine of an automobile or the like, air adjusted by an air conditioner within a cabin of the automobile or the like by an air cleaner 2AF and supplying the filtered air to the engine. Fuel consumption and harmful exhaust gas can be reduced by adjusting the temperature and humidity of the intake air.