An apparatus for monitoring a door with a 3D sensor which is able to detect distances to an object in the monitoring area as the third dimension, the 3D sensor being arranged in a housing, and immovably arranged transmission means for transmitting a measurement beam and receiver means for receiving a reflected measurement beam being provided in the housing, characterized in that provision is made of a control unit which is designed to obtain an item of information relating to the position of the apparatus, and in that the control unit evaluates the measurement beam on the basis of an item of position information.