To provide an opening/closing detection device which is downsized to improve mounting workability for a window or the like and design quality while actualizing excellent crime prevention.

On a frame body 2, a plurality of movable bodies 3, 4 are movably mounted which can open/close at least part of a space encircled by the frame body 2. Ferromagnetic bodies 15, 16 are provided on the movable bodies 3, 4. At least one of the ferromagnetic bodies 15, 16 is formed of a magnet. On the frame body 2, a magnetic detection element 21 is provided so as to be arranged in a magnetic circuit which is formed by the ferromagnetic bodies 15, 16 on the movable bodies 3, 4 when the movable bodies 3, 4 are closed.