To surely detect window opening by fraudulent activity such as to make it impossible to determine window opening and closing by using a strong magnet at the same time with the detection of window opening and closing.

A window opening/closing detection device includes a window opening/closing detection magnet, a hall sensor 46 for window opening/closing detection capable of detecting a magnetic level of positive polarity and negative polarity in accordance with the direction of magnetic force of a window opening/closing detection magnet and another strong magnet, a window opening/closing registration processing part 74 for registering a first determination threshold TH1 closer to a prescribed ratio zero point than a magnetic force level Vs by the window opening/closing detection magnet detected by the hall sensor 46 for window opening/closing detection in a window-closed state, and a second determination threshold TH2 farther than the prescribed ratio zero point, and a window opening/closing monitoring part 78 for determining window closing in the case that a magnetic force level by any magnet detected by the hall sensor for window opening/closing detection during a prescribed monitoring period is within the range between the first determination threshold TH1 and the second determination threshold TH2, and determining window opening in the other cases.