Abstract: Light is introduced to an inner surface of a fine tube, which is an object for measurement of the inner surface roughness, through a first end of the fine tube at a small angle with respect to a center line of the fine tube and the light emerging from a second end of the fine tube after the light is reflected by multiple reflections within the fine tube is measured to evaluate the roughness of the inner surface of the fine tube. Upon such measurement, the light to be introduced to the first end of the fine tube is converged once on the center line of the fine tube and then diverged again, and the polarization direction of the light to be introduced to the first end of the fine tube is set to an angle equal to or around 90 degrees with respect to the reflection surface of the fine tube. Thus, the roughness of the inner surface of the fine tube can be calculated in accordance with a result of detection of the intensity of emergent light from the fine tube by means of a photo detector.