A joining device suitable for a piece of furniture comprising at least two panels (11,12), said panels having seat (S1,S2) and said joining device having a compact leveller for parts of furniture and furnishing items, in particular between a first panel, such as a shoulder of a piece of furniture, and a second panel, such as a base or shelf of a piece of furniture, which must be moved towards each other according to an approach direction (d) in order to bring the edge of one panel abutted in a tightening position against a surface of the other panel, said joining device comprising a blocking group and a connection group assemblable on said panels (11,12), characterized in that a levelling group is also provided, insertable inside a same seat which receives the blocking group, wherein said seat is contained in the thickness of the panel in which the blocking group is applicable, said seat extending from an edge towards the interior of said panel (11,12) according to an axis (x) which is perpendicular to an approach direction (d) between said panels (11,12), said blocking group and said levelling group being manoeuvrable through two separate holes (42, 42'); the joining device comprising a casing (39',40') and a second outer casing (39,40); wherein said blocking group comprises a pinion-toothed crown bevel (41,43), which is housed in said casing (39',40') inside said second outer casing (39,40), wherein said second outer casing (39, 40) is positioned in said seat in the form of a hole of said shoulder, wherein said bevel, which is accessible from a first hole holing of said shoulder and from a hole of a half-shell of said casing, controls the movement of a grub screw suitable for being engaged on said connection group; wherein said levelling group comprises a leg; wherein said levelling group is positioned in said second outer casing (39,40) and comprises a cylindrical element provided with an internal threading which engages with a complementary outer threading formed on an upper outer end of said leg extending towards a floor; and wherein said connection group comprises a threaded end, an internally threaded bush and a pin, said pin being positioned in said threaded end, in said internally threaded bush, arranged in a hole or seat of said base, said pin providing, in its protruding part, a housing for a blocking grub screw of said blocking group. wherein said blocking group comprises a pinion-toothed crown bevel (41,43), which is housed in said casing (39',40') inside said second outer casing (39,40), wherein said second outer casing (39, 40) is positioned in said seat in the form of a hole of said shoulder, wherein said bevel, which is accessible from a first hole holing of said shoulder and from a hole of a half-shell of said casing, controls the movement of a grub screw suitable for being engaged on said connection group; wherein said levelling group comprises a leg; wherein said levelling group is positioned in said second outer casing (39,40) and comprises a cylindrical element provided with an internal threading which engages with a complementary outer threading formed on an upper outer end of said leg extending towards a floor; and wherein said connection group comprises a threaded end, an internally threaded bush and a pin, said pin being positioned in said threaded end, in said internally threaded bush, arranged in a hole or seat of said base, said pin providing, in its protruding part, a housing for a blocking grub screw of said blocking group.