A switch structure of a lamp unit mounted at an interior wall member, includes a switch member and a switch knob. The switch member includes a switch lever which slides in a first direction for turning on or off the lamp unit. The switch knob includes an engagement portion for engaging with the switch lever of the switch member, and the switch knob being slidably retained on a design member. The switch lever is inserted in the engagement portion of the switch knob in the first direction and in a second direction in which the design member is mounted to the interior wall member while the engagement portion is elastically deformed.