A fixing structure of an indoor lamp includes a lamp unit and an interior wall member. The lamp unit includes a design part arranged at an indoor side of the interior wall member and a functional part arranged at an outdoor side of the interior wall member and assembled with the design part through a lamp attachment hole in a state that the functional part holds the interior wall member therebetween. The functional part has a connector portion for connecting a connector. The design part has a regulating portion which forms a connector insertion portion together with the connector portion when the design part is assembled with the functional part. The regulating portion engages with the connector inserted into the connector insertion portion to regulate displacement of the design part in a disengagement direction from the functional part.