A switch device of a flashlight includes a tubular switch housing disposed in a barrel, abutting against a coupling ring mounted fixedly in the barrel, and having an outer circumferential surface formed with an engaging groove corresponding to a positioning groove in the inner surface of the barrel. The engaging groove has a depth that gradually increases toward an open end of the barrel. A switch unit is housed in the switch housing, and includes an operating member extending outwardly of the barrel through an opening in the switch housing and a through hole in the barrel. A wedge member engages the positioning groove in the barrel and the engaging groove in the switch housing, and has a thickness that gradually increases toward the open end of the barrel so as to prevent movement of the switch housing in the barrel.