The formation of the retaining member 42 at the forming of the main board 40 so as to be able to be cut off and the cutting-off from the main board 40 after the board forming of the main board 40 causes the retaining member 42 to be separated in a letterpress shape from the main board 40. In addition, on the main board 40, the earphone plug socket 44 is mounted with the plug-in inlet 44b facing the outer periphery, the slit is formed and the projected part 43a of the retaining member 42 is fitted into the slit as well. Further, the fitting of the retaining member 42 into the slit 46 causes the side edge 43b thereof to abut on a groove 44a at the back surface of the earphone plug socket 44.