PURPOSE: To increase arbitrariness in the place where an air conditioner is installed, to secure a large housing space for piping connection and to shorten the time to install the air conditioner by providing a cutout and a recess in the bottom surface of a mounting frame for air conditioner recessed in wall.

CONSTITUTION: The title frame 5 is constituted of a plurality of plates 5a in the longitudinal direction and a plurality of plates 5b in the lateral direction. One of the plates 5a in the longitudinal direction which forms a bottom surface of the frame 5 has a cutout 8 at a part of the back. By this, it is possible to mount an air conditioner even when a space between an inner wall and an outer wall is very narrow. Thereby restrictive conditions as for an installation place can be decreased, so that a living room can be used effectively. Further an increase of housing space for piping and a shotening of installation time can also be realized.