1. An image processing method comprising: obtaining a first spectral characteristic value in 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 a predetermined staining dye and to be observed; obtaining gradual second spectral characteristic values on the staining dye; and estimating an amount of the staining dye in the sample point on the staining sample by using the second dye spectral characteristic values on the staining dye based on the first spectral characteristic value.