To provide a fixing structure of an indoor lamp capable of improving installation strength of a lamp unit to an interior trim wall material without increasing the number of parts.

A room lamp 1 installed in a lamp installation hole 51 arranged in a roof trim 5 of a vehicle, includes a design part 2 arranged on the indoor side of the roof trim 5 and a function part 3 arranged on the outdoor side of the roof trim 5 and installed in a state of sandwiching the roof trim 5 by being joined to the design part 2 via the lamp installation hole 51. The function part 3 is provided with a connector part 36 for connecting a connector 7 connected to a wire harness 6 on the vehicle body side, and the design part 2 is formed with a design part side connector insertion frame 28 which forms a connector insertion part 40 together with the connector part 36 when the design part is installed with the function part 3, and regulates the design part 2 from being displaced in the separation direction from the function part 3, by engaging with the connector 7 inserted into the connector insertion part 40.