To provide an opening/closing device state monitoring device capable of eliminating a harmful effect of a surrounding environment in determining the occurrence of a predetermined phenomenon by processing an output of a photoelectric sensor, and to provide an opening/closing device.

This opening/closing device state monitoring device monitors the occurrence of the predetermined phenomenon of the opening/closing device in the movement of an opening/closing body, based on the output from the photoelectric sensor comprising a light projector and a light receiver. The monitoring device has a surrounding environment information acquisition means for acquiring information on the surrounding environment of the photoelectric sensor, and a surrounding environment change compensating means for changing a parameter for determining the occurrence of the predetermined phenomenon according to the surrounding environment information acquired by the surrounding environment information acquisition means. The opening/closing device is equipped with the opening/closing device state monitoring device.