PURPOSE: To improve S/N by moving and sending a light transmitting film material, casting measuring light on a plurality of mutually different parts of the surface of the film material in the intersecting direction, and receiving and detecting the light.

CONSTITUTION: A film-material sending means A has an unwinding roll 1 for unwinding the material 3 at the one-end side of a light transmitting film material 3 and a rotating take-up roll 2 for taking up and driving the material at the other end side. A returning-guide supporting means 4, which has guide rolls 4a, b, c, d and e for moving and sending the material 3 by one to several times of returning states, is provided. A pair of a single measuring-light emitting means 6 and a measuring-light detecting means 7 are provided at both end parts of the film 3 in the intersecting direction with respect to the surface of the material. Illumination light L1 is emitted from the emitting means 6. The incident light L. is transmitted at least twice so as to intersect and cross material 3, which is returned and sent. The output light L0 is received and detected with the detecting means 7.