1. An image processing device, comprising: a color information obtaining part obtaining at least a hue from color information of each pixel of a color image of a stained specimen; a detecting part detecting, from a hue distribution determined by plotting a hue of each pixel of the color image on a plane represented by a hue and saturation of a color space defined by the hue, saturation and intensity, a mode value of the hue and a hue area on the plane to which the hue distribution belongs; a range setting part setting, based on a most plotted area on the plane within the hue area to which the hue distribution belongs, a first straight line at a position rotated centering around an intensity axis by a first angle in a counterclockwise direction with respect to a reference line and a second straight line at a position rotated centering around the intensity axis by a second angle in a clockwise direction with respect to the reference line, the reference line connecting the mode value of the hue and the intensity axis, and setting a target range for a hue of cells to be observed among a plurality of stained cells of the stained specimen to include the mode value of the hue positioned between the first and second straight lines; a changing part changing color information of a pixel having a hue and being included in the target range by extending a hue distribution included in the target range to other hue area and extending the target range to not to include all hues on the plane in the target range by rotating the first straight line in the counterclockwise direction centering around the intensity axis with respect to the reference line and rotating the second straight line in the clockwise direction centering around the intensity axis with respect to the reference line; and an information converting part converting color information of a pixel having at least one hue not included in the target range into color information having a hue outside the extended target range.