A pair of slits (2a), a pair of first grooves (2b), a pair of second grooves (2c), and an engaging aperture (2d) are formed to the bottom plate (21) of an outer casing of the telephone set (1). An adapter plate member (3) has a base pate (3a), a pair of generally triangular plates (3b) standing on the base plate, a pair of elastic claws (3c) formed on the top edges of the triangular plates, and a mating tab (3d). One end of the adapter plate member is remobably and rotatably mounted at the front edge of the bottom plate. In a first condition where the telephone set is disposed with the bottom plate is not inclined, the triangular plates are inserted into the slits. In a second condition where the telephone set is disposed with an increased inclined angle, the claws are inserted into first grooves. In a third condition where the telephone set is mounted on a wall by the use of the adapter plate member. After the adapter plate member is removed from the bottom plate, turned upside down, and fixed onto the wall. Then, the tab is inserted and engaged in the aperture of the bottom plate, while each of claws is inserted into each of grooves.