Abstract: In a lamp body to which a switch knob is attached, an opening corresponding to the switch knob is formed. Opening has opposing sidewalls, and on each of the sidewalls, three grooves are provided. On one surface of a knob body of the switch knob, a pair of projecting pieces are provided spaced from each other by such a distance that allows the projecting pieces to be in contact with the sidewalls of the opening, and at each of the projecting pieces, a beam portion is formed. On each of the beam portions, a projection is provided on the outer surface, and stable engagement with the vertical groove is realized.