PURPOSE: To surely judge color even if a color mark on a work is soiled.

CONSTITUTION: An image pickup means 3 picks up an image of a color mark on a work and outputs the color information on every picked-up picture element. An arithmetic means 5 finds out the saturation, hue and lightness of each of picture elements of the color mark based upon the color information. An extracting means 7 extracts picture elements having the saturation higher than a previously stored prescribed threshold out of respective picture elements of the color mark. A judging means 9 judges the color of the color mark based upon the saturation, hue and lightness of the extracted picture elements. Since picture elements to which dirt or the like is stuck can be excluded, color can surely be judged.