First and second housings of a metal sheet are superposed over each other and fixed to each other by bolts to form an element receiving space. An antenna is fixed to the second housing by a bolt so as to be adjacent to the first housing. A hook section is fixed to the first housing. The hook section is inserted in a hole of the antenna, a step-shaped forward lower step section of the hook section is engaged with the peripheral edge of the hole in the antenna, and thus the first housing and the antenna are fixed to each other. The antenna is embedded in resin and covered by the resin.