A device for measuring depth of a groove formed on an article includes a light source for illuminating the article, a first detector for receiving light from a portion of the article having a groove and being illuminated by the light source, and for detecting a spectral reflectance in relation to the received light, a second detector for receiving light from the article illuminated by the light source and for detecting a spectral reflectance in relation to the received light, the second detector being effective to detect the spectral reflectance in a way not affected or substantially not affected by the groove of the article as compared with the detection by the first detector, and a depth detector for detecting the depth of the groove on the basis of the results of detection by the first and second detectors.