The invention provides an apparatus for monitoring a door with a 3D sensor, comprising: 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, immovably arranged transmission means for transmitting a measurement beam and receiver means for receiving a reflected measurement beam being provided in the housing, and 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. The apparatus is designed to be arranged at a base plane installation position and at least two other different positions. The installation positions are designed to be that whatever the installation positions are, the obtained installation directions are the same and the installations are matching with a housing such that an single direction of the at least two other different positions on the housing is determined through the installation device.