Recently, it was shown that the shape of lightning-generated induction magnetic field waveforms in the ELF frequency range is well comparable to that of lightning current waveforms. Therefore, charge amounts of any lightning discharges occurring within the area where the induction magnetic fields are measured can be easily estimated from ELF waveforms by quantitatively evaluating the relation between ELF waveforms and the current waveforms. In this study, we analyzed lightning current waveforms measured by a Rogowski coil installed at Mt. Ogkami and ELF waveforms measured at Onagawa observatory. Based on the quantitative comparison between the ELF waveforms and the current waveforms, empirical equations that enable us to directly convert from the magnetic field intensities into the current intensities and charge amount were obtained. Furthermore, using ELF waveform data obtained at Kuju station in Kyushu and the Japan Lightning Detection Network (JLDN) lightning data, peak current values and charge amounts for the lightning discharges occurring when severe downbursts occurred in the Kanto Plain were estimated by applying the empirical equations. Then, we newly found a clear feature showing that the time variation of charge amounts drastically changed just before the downburst onset.