1. An image processing method comprising: obtaining a spectral characteristic value at a sample point on a stained sample based on a value of a pixel of a stained sample image obtained by imaging the stained sample, the sample point corresponding to the pixel, the stained sample being stained with at least one staining dye; obtaining a plurality of reference spectral characteristic values for a single staining dye included in the at least one staining dye; and estimating an amount of the staining dye at the sample point on the stained sample by using the plurality of reference spectral characteristic values and the spectral characteristic value at the sample point on the stained sample.