To obtain a magnetic force level of adequate resolution by widening an allowable installation range of a magnet to be separately arranged to a device built-in magnetic sensing element.

A first window opening and closing detecting Hall sensor 46-1 and a second window opening and closing detecting Hall sensor 46-2 are arranged in a window opening and closing detecting device with respect to the magnet arranged at another window side which performs relative movement to a window where the device is installed. A sensor selection part compares the magnetic force levels detected by first and second window opening and closing detecting Hall sensors 46-1 and 46-2, and selects the Hall sensor most effective in opening and closing of monitoring. A window opening and closing monitoring part determines opening or closing of the window by comparing the magnetic force level detected by the selected Hall sensor with a predetermined determination threshold and wirelessly transmits a determination result.