A technique for estimating an unsampled color data on the basis of plural color data sampled by a line image sensor. The sensor includes at least two types of color filters such as a red filter and a green filter. Those filters are alternately arranged pixel by pixel in a line of pixels so that A-type color data which corresponds to green data is sampled from a target pixel and plural B-type color data which correspond to red data are sampled from neighbor pixels neighboring the target pixel. An unsampled B-type color data of the target pixel is estimated by synthesizing the sampled A-type color data of the target pixel and the sampled B-type color data of the neighbor pixels.