PURPOSE: To enable a high-speed scanning by/using a transmitted light, by a method wherein a sample conveyed in the direction perpendicular to the direction of deflection is scanned by a light beam deflected in a reciprocating manner, and a beam collector and an optical detector are provided on the side opposite to the incident side of the light beam.

CONSTITUTION: A light beam B from a light source L is deflected in reciprocation between points Q and Q' on a plate P by swinging a mirror (m) through an angle θ. A beam collector C collecting transmitted lights TWT' is provided on the back side of the plate P, and the lights are reflected repeatedly on the inner surface of the collector C and made to fall on an optical detector D. A sample is placed on the plate P and subjected to deflection scanning sequentially by transferring the plate P perpendicularly to the direction of deflection. Prior to measurement, the plate P is shunted from the light beam, the mirror (m) is made to swing so as to find the relationship between the position of a spot of the beam and the output of the optical detector D, the relationship thus found is stored in a control unit not shown in the figure, and the result of measurement is compensated based thereon. By this method, high-speed scanning is performed by using a transmitted light, and thereby an accurate quantitative measurement is enabled.