PURPOSE: To suppress the effect of the scattered light caused by the surface roughness of a material under inspection to a minimum and to detect the scattered light from a foreign matter readily by specifying the emitting direction of detecting light on the surface of the material under inspection and the detecting direction of the scattered light.

CONSTITUTION: A light emitting means 3 is arranged so that the light is emitted in the direction in parallel with the rolling direction of a base material 1. A slant angle θ of detecting light from the light emitting means 3 with respect to the base material 1 is usually selected at 5 - 25 degrees so as to increase the scattered light from a foreign matter. A detecting means 8 for detecting the scattered light from a detecting position is provided so that the optical axis is inclined to the side where the means 3 is arranged with respect to the base material 1. The slant angle α is set at 60 degrees or less. The scattered light (noise) caused by the irregularities on a material under inspection itself is suppressed to a minimum by specifying the angle of the optical axis, and the scattered light caused by the foreign matter can be positively detected.