Structure of furnishing element, in particular of a bookcase, having a first upright having an outer surface and an opposite inner surface; at least one inner shelf delimited by a first lateral end having a first abutment surface; at least a first engagement portion retained in the at least one inner shelf; at least a first cantilever bar adapted to be mounted transversely from the outer surface of the first upright, the first cantilever bar including an abutment surface adapted to abut on the outer surface of the first upright; at least a first threaded connection element adapted to connect the first cantilever bar and the first engagement portion so that the abutment surface of the cantilever bar and the first abutment surface of the at least one shelf are drawn respectively against the outer and inner surfaces of the first upright on opposite sides to the first upright.