A switch comprises a room light case, a switch knob arranged inside an aperture portion of the room light case, guide grooves provided on an inner circumferential face of the aperture portion of the room light case, and guide ribs provided on an outer circumferential face of the switch knob. The switch knob moves for a switch stroke part in a depth direction of the aperture portion, from a waiting position to a pressed-down position by a pressing down operation. The guide grooves are provided in a range deeper than a depth size of the switch stroke from an opening edge located on the switch case surface side of the aperture portion.