The invention relates to a device for automatically raising or lowering a window covering provided with cords, comprising an elongate housing (2') in which is arranged at least one rotatable shaft (3) provided with at least one winding roller for winding up or unwinding the cords of the window covering. The device further comprises a motorized drive (8) for rotatingly driving the shaft and at least one mechanical transmission between the motorized drive and the rotatable shaft which comprises a worm wheel transmission. The drivable element (7B') of the worm wheel transmission is arranged on the rotatable shaft and the driving element (7A') of the worm wheel transmission is connected to the motorized drive. The motorized drive and the mechanical transmission can be accommodated in a second housing (12'), which second housing can be releasably coupled to the elongate housing.