To provide an air filter device for air conditioner for preventing generation of a chattering sound in operation by eliminating the looseness of a prefilter in a filter grille.

An air cleaning filter (f) is constituted so as to lock the prefilter 10 with a filter locking claw 3 by housing the prefilter 10 on the reverse of the filter grille 6, and requires to eliminate the looseness of the prefilter 10 housed in the filter grille 6 for preventing generation of the chattering sound. Then, a frame rib 8 of the prefilter 10 is arranged in a position opposed to a grille rib 5 of the filter grille 6, and is constituted so that a projection part 8a is arranged in an almost central part of an opposed surface with the grille rib 5 on the frame rib 8. When locking the prefilter 10 by the filter locking claw 3 by housing the prefilter 10 in a frame body 4, tension is applied to the prefilter 10 by pushing the projection part 8a against the grille rib 5.