PURPOSE: To make installation of the captioned casing easy, by a method wherein a larger opening than the inside measurement of both frame side plates is provided on the casing, a grille holder part perpendicular to a side plate and a horizontal bent part are provided on the circumference of the opening and both the bent part and a bent part of the frame are secured with bolts.

CONSTITUTION: The front 10 of a casing 9 is punched and bent so that a blowing down grille containing opening part 14 larger than the inside measurement between both frame side plates 2, a grille holder 12 perpendicular to the frame side plates 2 and a fixing part 13 in a horizontal direction are formed. Then, a rise part 16 bent upward is provided on a receded location than the front of the casing 9, on which holes 15 are provided. Upon touching of the opening 14 of the casing 9 to a frame 1, front bent parts 2a of the frame side plates 2 are fixed to the fixing part 13 with bolts and a rise part 16 and the rear edge of the lower face are connected by blots through holes 15 and 18 with each other respectively. Then, a blow down grille not illustrated in a sketch is installed on t he opening 14 detachably and attachably. With this, assembly without using special parts can be done easily.