We developed an inverse Radon-transform method to reconstruct slip velocity intensity (SVI) on the fault plane by using near-field seismograms. The observed source time function is approximated to the integration of the product of the SVI and the isochrone velocity along the isochrone, the shape of which depends on the fault plane-observation point geometry, and the rupture and wave velocities. Our approach combines the back-projection method with a windowing procedure. Our approach has the advantage of not requiring a priori assumption of faulting area over the previous inversion studies of seismic source processes. Some simulation models used to test the applicability of our method have shown that it can recover well the SVI distribution. This method is applied to the 1980 Izu-Hanto-Toho-Oki earthquake. Our result shows a heterogeneous SVI distribution over the fault plane. Relatively large values of SVI were found in the north and center areas of the fault plane from the aftershock occurrence and the regions correspond to those region with less aftershock occurrence. The heterogeneous rupture effectively radiating high-frequency seismic wave is restricted to a smaller region than the inferred fault plane.