A method for the measurement of the radial flux induced by the electrostatic fluctuations was developed by use of gold neutral beam probes (GNBP). Beam probes were useful devices for the measurement of density and the potential fluctuations in core plasma. The drift wave measured by a GNBP was growing with the finite phase difference between the density and the potential fluctuation. It was found that using the GNBP, the measured drift wave caused the density and the potential profile to be relaxed through the radial particle flux. Since the plasma difference of the electrostatic fluctuations would cause the radial flux, the measurement of the phase difference had been one of the key parameter for the radial confinement.