The Heavy Ion Beam Probe (HIBP) diagnostics on T-10 tokamak was modified to measure the local values of the plasma potential in the ECRH plasma. The Tl'ion beam with the energy up to 240 keV and intensity about a few dozens pA was used. The clear fall ofpotential in the range of-200 -600 V was observed in the outward EC resonance during spontaneous improvement of plasma confinement. The fall down of the potential accompanies the fall of Do and the density increase. The time evolution of the potential profile demonstrates formation of the thin (about I cm) area near the limiter with strong electric field E, (-300 V/cm). All this means that the H-mode with ECRH as the sole source of the auxiliary heating in circular limiter tokamak was first obtained. The comparison with DIII-D and JFT-2M data demonstrates the similarity of typical features of the L-H transition. The measurements during the electron internal transport barrier (EITB) formation show the appearance of the local negative potential well about I kV in the region of the barrier. The local transient values of the E, reach -1000 V/cm.