1. An image input system comprising: an acquiring part for acquiring color image data of a subject; an object color estimating element for estimating first object color component data of a first subject by using first color image data of said first subject acquired by said acquiring part under predetermined conditions; an illumination light estimating element for estimating a spectral distribution of stationary light on the basis of second color image data of said first subject acquired by said acquiring part under said stationary light and said first object color component data; and a calculating element for calculating a conversion matrix for converting third color image data of a second subject acquired under said stationary light to second object color component data of said second subject by using said spectral distribution.