PURPOSE: To accurately detect the chrominance of a register mark color, making the replacement of color filters unnecessitated, by a method wherein, a register mark is color-separated into three primary colors i.e. red, blue and green, and their density are judged while being added with a signal from a margin blank.

CONSTITUTION: A light source lamp 8 illuminates a yellow, a red, and a blue register marks 5, 6, and 7, the components in the infrared region are removed by a heat-ray absorption filter 9, and subsequently, the color-separation is performed by red, blue reflection dichromic mirrors 10, 11. The red, blue and green colors issued from red, blue and green transmissive filters 12, 13, 14 are received by photoelectric conversion elements 15b, 15c, and 15d while the light reflected from a margin black 3 of printing paper is received by a photoelectric conversion element 15a, then the signal therefrom is inverted-amplified by an inverting amplifier circuit 17. By adding the values thus obtained by an adding circuit 18, the generated output becomes chrominance value of the three primary colors. The chrominance value and the output of a threshold value generating variable resistance 19 are compared with each other resulting in judging signal. Thereby, the method mentioned herein ensures correct detection with regard to a density chrominance degree, and even if a printed color varies, the replacement of filters is unnecessitated.