A vehicle interior illumination lamp for mounting at an opening portion formed in a roof panel (interior member) of a vehicle includes a function part including a bulb (light source), and a switch unit for supplying and interrupting electric power relative to the bulb, and a design part including a housing for mounting at the opening portion in the roof panel, a retaining mechanism securely engaging the function part with the housing, and a switch knob engaged with the switch unit. The retaining mechanism includes a retaining member which cooperates with the switch knob to hold the switch unite therebetween.