Potential and density/temperature fluctuations at the L-H transition aremeasured by a heavy ion beam probe (HIBP) on JFT-2M. It has been observed thatthe time scale of potential change is as fast as 10-100 µswhen the input power (Pin) is larger than the L-H threshold power(Pth). When Pin = Pth, the confinement is improved stepby step, with sawteeth crashes accompanied by a decrease of potential. After afew sawteeth crashes, the potential drops rapidly to the level of the ELM-freeH-mode. At that time, the density/temperature fluctuations are suppressedsimultaneously. The observations of the temporal behaviour of the potentialand fluctuations in such an L-H transition are reported.