The invention relates to a device for automatically actuating a door, in particular a vertical door, which aims to replace to a great extent known signal generation devices or control signal sensors for opening the door or for the security monitoring of the closing motion of the door. This is achieved by a detector of a security system, which is configured as a scanner detector and operates according to reflective principles, for monitoring an area by allocation to one of the ground level aprons of the door, said detector being mounted in the vicinity of a lateral border of the door opening, at a height, which is adjusted to the maximum height of objects supported on the relevant apron that are not to be registered as obstacles. The scanning beam of said detector scans a horizontal fan-shaped area, which covers the relevant apron and has a low vertical depth, determined by the depth of the scanning beam, as the monitoring area. The radial dimension of said fan-shaped area, which is determined by the maximum measuring range of the scanning detector, is selected according to the size of the relevant apron.