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A digital image processing technique using the Fourier transformation in the spatial domain and the Wiener filter has been developed. This method is applied to the measurement of the accumulated charge on the opposite sides of a planar insulator using an electrostatic probe. In the spatial frequency domain, the relational equations between the probe outputs and charge distribution are given by the first order simultaneous equations of two unknowns. In the course of the estimation of the charge distribution from the probe outputs, the Wiener filter is used to reduce the influence of the experimental noise. The spatial resolution of this method is also quantitatively discussed from an analysis in the spatial frequency domain. Using this technique, the distribution of the surface charge density on both surfaces of an insulating plate is measured immediately after the occurrence of a surface discharge. When the probe scans both sides of the plate, the charge distribution is estimated with a spatial resolution of 1.6 mm. In the case that the probe scans from only one side of the plate, the two-layered charge distribution is estimated with a limited resolution.