To accurately detect various surface detects of a substrate or the like in a noncontact state by changing the diffracting direction of diffracted light from the surface of a subject with regard to a light-receiving system in accordance with the pattern information of diffracted light and scattered light.

A projection screen surface 6 has a shape capable of taking in the diffracted light and scattered light produced in accordance with the surface condition of a subject. Image pickup optical systems 7R and 7L pick up the patterns of the diffracted light and scattered light projected on the projection screen surface 6. Image pickup means 8R and 8L are used for observing the pickup images of the pattern. A variable means 4 changes the diffracting direction of the diffracted light from a subject surface with regard to a light- receiving system, in accordance with at least one light-receiving system arranged outside or in the vicinity of the projection screen surface 6 and with the pattern information of the diffracted light and scattered light obtained by the image pickup means 8R and 8L. Thus, the distribution of diffracted light and scattered light in a wide range can be known, so that the kinds of defects on a substrate can be discriminated.