when step s 30 is repeatedly executed five times color component data of b c g y and r can be obtained substantially at the same position on the original in this example the holder is continuously driven in the subscan direction alternatively subscan drive may be performed while repeating movement and stop in units of lines in this case the image for five lines is read while the holder 62 is at a stand still so the color component data at the same position can be obtained when the value of the counter nc exceeds 4 the flow advances to step s 33 that is when all color component data of b c g y and r are acquired the flow advances to step s 33 in step s 33 color calculation for one line image data is executed a color image signal processed by a device such as a color display or a color printer generally comprises three color signals of r g and b which represent the levels of color components having prescribed wavelengths of r g and b respectively on the other hand the signal obtained by processing in step s 30 has five color components of b.