PURPOSE: To attain labor saving and rationalization by coding the output of reflected light from a plane of a body to be inspected into a binary signal, and deciding whether the total number of bits having logic corresponding to the reflection from the plane is larger than a specific value or not.

CONSTITUTION: Projecting devices 3a and 3b project light upon the entire top surface of the body 1 to be inspected which is moved by a conveying device 2 in parallel. A photoelectric converting device 4 which incorporates a lens whose focal length is set small enough to focus only an image of the top surface of the objective body 1 makes a scan at right angles to the moving direction of the objective body 1. The moving direction of the objective body 1 coincides with one of ridge directions of ridges to be inspected preferably. While the output of a position detector 6 indicates that the objective body 1 is at a position to be inspected, the entire top surface of the objective body 1 is irradiated by the projecting devices 3a and 3b and the photoelectric converting device 4 makes the scan.