Abstract: In this paper, based on correlative analysis between observed SPAevents for VLFsignals and the flux density of solar X-ray bursts, a quantitative relation between the flux of the solar X-ray bursts and height variations of the ionosphere at the time of SPAevents is found. From this relation it is possible to estimate the order of flux density of the solar X-ray burst on the ground using SPA's data, while direct observation data are difficult to attain at the ground level because of absorption of the atmosphere. As an example, a SPAevent of VLFsignals occurred on June, 1982 is calculated. The calculation results are compared with experimental data. It seems that this result is fairly good.