An air filter (14) having a plurality of unit filter frame members (22) each of which includes a filter piece (3) and which are coupled foldably through hinge portions (23). A guide portion (32) is provided to have at least a pair of boss portions (25) made to project from each of opposite side surfaces (35) of the unit filter (14), so that the guide portions (32) are provided on the opposite side surfaces (35), and are respectively inserted into and guided by guide grooves (26) provided at the place where the air filter (14) is to be installed. <IMAGE>
