PURPOSE: To contrive to make parts common, lower cost down, facilitate mounting works, and concurrently enhance workability.

CONSTITUTION: A room lamp mounting member is fixed to a roof reinforcing member interposed between a roof panel and a molded ceiling 3 through an opening hole formed in the molded ceiling 3, and a room lamp main body is then mounted to the aforesaid room lamp mounting member. A projected piece 3a extended out of an opening hole peripheral section 4a is provided for the opening hole formed in the molded ceiling 3, the room lamp mounting member 5 is engaged with the projected piece 3a so as to be supported, and the room lamp mounting member 5 is threadedly fixed to the roof reinforcing member 2 thereafter.