The present invention relates to a flat-running device intended for equipping a mounted assembly without an air chamber for a motor vehicle, and to such a mounted assembly incorporating this device.This device (30) comprises a wheel rim (10) with a plurality of blocks (10a and 10b) and a tire (20) comprising beads (21 and 22) mounted against flanges (12 and 13) of the rim, the device comprising: a structure (31) for supporting the tire, which structure is to be mounted around the rim and is divided into ring sectors (32) in the form of an arc of a circle, surmounted by a casing (34) which is intended for supporting the tire when the latter is running flat and which has two projecting lateral portions (34b) partially covering lateral faces (32b) of the structure, and means (33) for locking the beads against said flanges, which connect the annular supporting structure to the beads. According to the invention, these lateral portions incorporate means (34c) for holding the sectors, opposing the axial displacement of the latter under lateral forces in the event of a shock during running in the inflated state or in the event of running flat, so that these sectors remain surmounted by the casing.