Abstract: This invention relates to a bayonet-type coupling device of the back-cover and the middle of a waterproof shaped watch-case, in which said middle presents a hollow comprising an annular groove in its internal wall and said back-cover comprises a cylindrical or prismatic annular wall provided with at least two ledges, and in which the sizes of said annular wall and of said hollow are such that, for at least an angular position of the back-cover with regard to the middle, they are free, the annular wall of the back-cover being placed within the hollow of the middle, and that, for at least another angular position, they are coupled by engagement of at least a part of the ledges in the annular groove, characterized by the fact that said angular coupling position is defined by the contact of a part of the external surface of the annular wall of the back-cover with the internal surface of the hollow, both these surfaces having the same shape and the same sizes, so that the external surfaces of the middle and of the