PURPOSE: To measure at a high accuracy even though the measuring surface forms of a work to measure and a standard sample are different, by regulating the sample measuring position and the measuring surface of the work to measure coincide constantly.

CONSTITUTION: The light radiated on a sample measuring position 3 by a fiber 2 for projection is reflected by a work to measure at the position 3, and the quantity of light of the received light by a fiber 4 for receiving is measured by a spectroscope or the like. In this case a movable attachment 7 maintains the fibers 2 and 4 at a constant angle in a holder 11, and the parts contacting with the work to measure are expanded and contracted. By regulating the expansion distance L by a regulating handle 8 through the combination of gears, for example, the distance between the fibers 2 and 4 maintained at a constant angle, and the measuring surface of the work to measure is regulated. Consequently, even when the form of the measuring surface of a work to measure is different from the form of the measuring surface of a standard sample, the absolute reflectance of the measured surface can be measured at a high accuracy.