A seat belt buckle fastening detector 40, which detects the existence of seat belt fastening, includes a printed circuit board 41 including a normally-closed contact NC, a normally-opened contact NO, and a common contact COM. A slider 42 electrically connects the normally-closed contact NC and the common contact COM, or the normally-opened contact NO and the common contact COM. The common contact COM is formed on one surface 41 a of the print circuit board 41, and the normally-closed contact NC and the normally-opened contact NO are formed on the other surface 41b of the print circuit board 41.