A primary printed circuit board (PPCB) is put on a plurality of leg rods integrally protruding from a bottom plate portion, a central insertion hole, which is one of the three insertion holes formed in the three corners of the PPCB, is engaged with an engaging pin integrally protruding from an upper surface of one of the leg rods, a screw is screwed into a screw hole of one of the leg rods through one of the insertion holes opposed to each other along a diagonal line, a holding rod is integrally protruded from a roof plate portion being opposed to one of the insertion holes opposed to each other along the diagonal line, a holding pin protruded from a lower surface of the holding rod is engaged with an engaging hole of one of the leg rods through one of the insertion holes, and a small gap is formed between a lower surface of the holding rod and the PPCB.