To observe a rolling interface while a work roll is being rotated and driven, by guiding illumination light to an observation container being provided near the inner-periphery surface of the work roll of a rolling device, and providing an optical mirror cylinder for guiding signal light being reflected from the rolling interface to a detection means in the work roll.

An optical mirror cylinder 14 is provided so that it matches rotary axial lines in holes 1b and 1c being formed in a work roll 1. When it is detected that an observation window 10 contacts a rolling interface 3a of a rolled work 3 according to a timing signal from a rotary encoder 24, illumination light being emitted from a light source 6 is reflected from a mirror 12 and passes through the hole 1c, is transmitted through the observation window 10, and is applied to the rolling interface 3a. Then, reflection signal light from the rolling interface 3a is transmitted through the mirror 12 and is reflected by a mirror 18, an image is formed on the light reception surface of a CCD camera 8 by an image-forming lens 9, and its output is displayed by an image display means 23, thus obtaining various kinds of data at a transitional or dynamic process regarding the rolling interface 3a.