A switch body is mounted on a sub-board, the sub-board is held, by fastening it with screws together with a protective plate that protects a surface of the sub-board on the opposite side of a mounting surface of the sub-board, in a holding wall formed integrally with a jack holder such that the mounting surface of the sub-board is disposed facing a mounting surface of a main board, and an operation portion of a push button is allowed to project to the outside of a casing from a square hole disposed in the jack holder.