A mechanical connection system for connecting a prefabricated concrete slab to a steel beam, the slab having at least one recess that passes through its thickness and the cross-section of which decreases progressively from the upper surface of the slab to its lower surface by which the slab bears on an upper surface of the beam, the beam comprising, for each of the at least one recess, at least one dowel-type connector secured to the beam and extending in the respective recess from the upper surface of the beam, the at least one connector being threaded at least over part of its length, the connection system comprising: - at least one frustoconical intermediate part having, along its axis, a through hole by which the intermediate part is able to be slipped onto a respective connector, with the small circular base of the intermediate part facing the upper surface of the beam, - for each of the at least one intermediate part, attachment means for attaching the intermediate part to the connector by screwing on the threaded part of the connector, - for each of the at least one intermediate part, a frustoconical sleeve intended to be placed around the intermediate part in a recess, the connection system being characterised in that: - each of the at least one intermediate part consists of a frustoconical metal nut, the through hole of which is threaded over at least part of its height, said thread making up said attachment means, - each of the at least one sleeve is made of metal, - each of the at least one nut and each of the at least one sleeve are dimensioned to be assembled to one another by a conical connection, and configured so as, once assembled by conical connection, to define, around the axis of the through hole of the nut, a free space for connector flexure, the free space being delimited in the upper part by the nut, in the lower part by the imaginary plane to which the small circular base of the at least one sleeve belongs, and laterally by one among the at least one nut and the at least one sleeve, and - the connection system further comprises, for the or each recess, a metallic cage able to be placed in a respective recess and in contact with the walls of the latter, over the entire thickness of the slab, the cage to this end having securing means (40a, 40b, 41) intended to be embedded in the concrete during pouring of the slab. - at least one frustoconical intermediate part having, along its axis, a through hole by which the intermediate part is able to be slipped onto a respective connector, with the small circular base of the intermediate part facing the upper surface of the beam, - for each of the at least one intermediate part, attachment means for attaching the intermediate part to the connector by screwing on the threaded part of the connector, - for each of the at least one intermediate part, a frustoconical sleeve intended to be placed around the intermediate part in a recess, - each of the at least one intermediate part consists of a frustoconical metal nut, the through hole of which is threaded over at least part of its height, said thread making up said attachment means, - each of the at least one sleeve is made of metal, - each of the at least one nut and each of the at least one sleeve are dimensioned to be assembled to one another by a conical connection, and configured so as, once assembled by conical connection, to define, around the axis of the through hole of the nut, a free space for connector flexure, the free space being delimited in the upper part by the nut, in the lower part by the imaginary plane to which the small circular base of the at least one sleeve belongs, and laterally by one among the at least one nut and the at least one sleeve, and - the connection system further comprises, for the or each recess, a metallic cage able to be placed in a respective recess and in contact with the walls of the latter, over the entire thickness of the slab, the cage to this end having securing means (40a, 40b, 41) intended to be embedded in the concrete during pouring of the slab.