Abstract: The auxiliary device (1) for the installation of plate-shaped products (M) for covering floors and/or walls includes: a first element (2) having: a substantially flat base body (3) for supporting two or more adjacent plate-shaped products (M) and a gripping body (4) which is associated substantially at right angles with the base body (3), the base body (3) being suited to support the plate-shaped products (M) from opposite sides of the gripping body (4); a second element (6) associable with the gripping body (4) for fastening the plate-shaped products (M) between the base body (3) and the second element (6) to define the coplanarity of the plate-shaped products (M); and screw connection elements (7) suited to allow the coupling by screwing of the second element (6) on the first element (2) around a rotation axis (R) substantially at right angles to the base body (3).