To highly accurately judge the achromatic color image of original images without depending on a read variable power rate.

In the case of reading the original image by the respective line sensors of RGB provided with a prescribed interval in a sub scanning direction, a read speed in the sub scanning direction is controlled corresponding to a set variable power rate. At this time, since a space of an offset line number corresponding to the variable power rate is present between the respective line sensors, it is required to delay image data from the line sensor of a certain color component by the offset line number to the image data of the other line sensor. The offset line number does not always become an integer. A judgement area in (a) is used in the case that the decimal part of the offset line number is close to 0 and an achromatic color judgement area is enlarged as in (b) when the decimal part is closer to 0.5.