To clean air without changing the existing air-conditioning facilities of old building.

For air conditioning, an air filter 1 is directly mounted to an air outlet for blowing off air to the inside of a room in a building. An attachment/detachment mechanism 5 corresponding to existing fitment is provided in the air filter 1 for fitting to the existing air outlet of an old building. A straightening plate for diffusing blown air to a periphery is mounted to a filter medium.