An interlocking groove is formed on the rear upper surface of a chassis part so as to be exposed outside. A mounting hole opened backward is formed in an attachment portion. A first cover member includes a pivot shaft portion pivotably mounted to the mounting hole, and a first interlocking protrusion allowed to be interlocked with the interlocking groove when the pivot shaft portion is mounted to the mounting hole and the first cover body is pivoted with respect to the axis of the mounting hole. A second cover member is configured to cover the surrounding of the first interlocking protrusion interlocked with the interlocking groove in the lower side of the rear part of the first cover member by a retaining portion disposed on the upper part of the second cover member.