The case 12 is structured by an upper case 14 and a lower case 16, which are made of a synthetic resin such as polycarbonate (PC) or the like, being joined together by ultrasonic welding, screws or the like in a state in which a peripheral wall 14B, which is erectly provided at the peripheral edge of a top plate 14A, and a peripheral wall 16B, which is erectly provided at the peripheral edge of a