Installation apparatus consisting of a wall plug and a holder which can be screwed therein for installing flush-mounted devices

An installation device for installing flush-mounted fixtures (4) on/in a wall (5), wherein the installation device comprises a wall plug (1) and a retainer (2) that can be screwed therein, characterized in that the wall plug (1) has a sleeve-shaped body extending along a first longitudinal axis (10) that has a first free end (11), an opposite, second free end (12), and a securing section (13) lying therebetween, wherein a first front end limit stop (110) is provided on the first free end (11) of the wall plug (1), and wherein a first tightening element (120) is provided on the second free end (12) of the wall plug (1), in that the retainer (2) has a sleeve-shaped body extending along a second longitudinal axis (20) that has a first free end (21) and an opposite, second free end (21), wherein a second limit stop (210) is provided on the first free end (21) facing toward the second free end (22), and wherein a second tightening element (220) is provided on the second free end (22), wherein the first limit stop (110) and the second limit stop (210) form a pair of limit stops, and wherein the first tightening element (120) and the second tightening element (220) form a pair of tightening elements, wherein the securing section (13) is configured such that a relative rotation between the wall plug (1) and the retainer (2) in a first direction of rotation leads to a reduction in the distance between the first free end (11) and the second free end (12) of the wall plug (1), and that a relative rotation between the wall plug (1) and the retainer (2) in a second direction of rotation, opposite the first direction of rotation, leads to an increase in the distance between the first free end (11) and the second free end (12) of the wall plug (1).