PURPOSE: To make it possible to accomplish a unit mounting work at an arbitrary period of time regardless of a ceiling boarding engineering work by locating and forming inwardly the upper part of one side surface opposing another side surface in which a duct connecting port of a unit case is protruded.

CONSTITUTION: To upper side portion 10b-1 on a left side surface 10b confronting the duct connecting port 10e of a unit case 10 is shrunk inwardly from a lower side surface 10b-2 a dimension T equal to a projecting amount T of the duct connecting port 10e. An opening 13 preformed in a ceiling board 12 has a dimension which is slightly larger than the lower side portion of the unit case 10 and enough for inserting the same. The case 10 is inclined and a part of a flange 10g at the upper end of the case 10 is passed through the opening 13. Thereafter, the center L1 of the case 10 is made coincident with the center L2 of the opening 13, and the lower side part of the case 10 is passed through the opening 13.