In a fastening device for attachment to a mounting rail, comprising a toggle element for introduction into a mounting opening and engagement on the back of edges of the mounting opening of the mounting rail, a bracing device that works together with the toggle element, especially a clamping bolt, for clamping the fastening device on the mounting rail, a contact plate for outside contact on the mounting rail that has a pass-through opening for at least one part of the bracing device, a spring as compression and torsion spring, a holding device between the toggle element and the contact plate for holding the toggle element in the braced position, whereby during loosening of the holding device, the toggle element may be transferred by the spring from an introduction position to a braced position in engagement with one of the edges of the mounting opening.