By condition of atoms' concentration constancy similar to observing of the diffusion-induced grain-boundaries (GB) migration (DIGM) in single-phase metals and alloys, taking as an example the model system-gas of A sort atoms divided by a wall into volumes V-1 and V-2 there are thermodynamically analyzed the causes of pressure-difference occurrence at a wall (Delta P = = P-1 - P-2 where P-1, P-2-the pressures in bulks with volumes of V-1 and V-2, respectively)-the driving force of wall moving (G = Delta P not equal 0), which is induced by diffusion of B sort atoms along the wall and into the bulk (from a wall). The obtained conclusions for a model A-B-gas system are used in DIGM thermodynamic analysis. As noted, by decreasing of the free energy system, the DIGM driving force can arise due to the internal-energy increasing caused by arising (or amplification) of interaction of migrating GB sites with other GB sites and (or) with bulk defects (including generated during the DIGM process). As shown, the alloying in the GB migration region is not the cause of GB migration during the DIGM, and is a corollary of the second law of thermodynamics. The obtained conclusions concerning thermodynamic conditions of the DIGM observation do not contradict to the experimental results of DIGM investigation.