The device comprises a disposable element (10) and a wedge (12) both preferably of a cheap plastics material. The disposable element (10) has a T-shaped cross-section with a central portion (14) and at least two opposite yielding arms (16). The wedge (12) can be inserted in the hole (18) and has a flat levelling surface (24). The disposable element (10) is intended to be placed with its arms (14) under two consecutive flooring modules and immersed in the adhesive (mortar or resin) for fixing the modules to a floor foundation, and with its central portion (14) extending above the upper faces of the modules through a gap. One edge (20) of the hole (18) is arranged so as to be above the upper faces of the modules. The wedge (12) can be inserted in the hole (18) between this edge (20) and the upper faces of the modules with its flat levelling surface (24) in engagement with the upper faces so as to clamp the two modules between the wedge and the yielding arms (16).