The invention relates to a flat running device to be fitted on a tubeless mounted assembly for an automobile, and to such a mounted assembly including said device. The flat running device (30) of the invention comprises: an annular bearing structure (31) comprising a ring having a radially outside bearing face (31a) for bearing a flat-running tyre cover (20), and at least one tightening belt (32) surrounding the ring so as to maintain the same substantially in contact with a running rim (10), at least one radial reinforcement (31b) being formed in the bearing face, the tightening belt(s) being applied on said reinforcement and being recessed relative to the bearing face. According to the invention, the bearing structure includes a means (33) for locking it in this hollow (11) by mounting the same so as to bear against a side wall (11b) of the hollow, a radially inner face (31c) of the locking means being adapted so as to conform to the axial profile of the bottom (11a) of said hollow.