To mold a bell mouth efficiently in terms of time and cost to easily mold a front panel with the bell mouth in one piece and at the same time, easily mold a panel including a discharge grille.

The bell mouth 13 is integrally molded in plastic with a cylindrical rib 13' with cuts 13a' formed vertically in a front panel 10 without molding the bell mouth 13 into a curved shape from the beginning. After molding, the rib 13' is forcibly expanded into a curved shape using a heated bell mouth- shaped jig 20 to obtain the bell mouth 13. Thus it is possible to realize the integral molding of the front panel 10 and the bell mouth 13 easily. In addition, the front panel 10 including a discharge grille 12 can be molded.