A main circuit board (27) is mounted on a main body (2). A switch circuit board (20) is held in a mounting portion (13) such that switches (21 and 22) face upwards. As the switch circuit board (20) is held in the mounting portion (13), positioning projections (14a and 14b) formed on a bottom wall (14) are pierced through positioning holes (16a and 16b), respectively, and thus the positioning of the switch circuit board (20) inside the mounting portion (13) can be easily carried out. A switch cover (10) is provided above the switch circuit board (20) held in the mounting portion (13). A plurality of projections (25a, 25b, 25c and 25d) projecting downwards from the inner surface of the switch cover (10) are brought into contact with the upper surface of the switch substrate (20). As these projections press the upper surface of the switch substrate (20), the up-and-down rattling of the switch circuit board (20) inside the mounting portion (13) can be prevented.