PURPOSE: To detect a pattern with high accuracy by providing a third brightness information calculating means which compares first brightness information with second brightness information to obtain as the third brightness information the information converted to zero or sufficiently small value from the brightness in a region having the brightness smaller than the second brightness information among the first brightness information.

CONSTITUTION: First brightness information obtained from an image signal is compared with second brightness information obtained from a specified region having no pattern. A brightness value in a region having the brightness smaller than the second brightness information among the first brightness information is converted to zero or sufficiently small value to provide third brightness information. The center of gravity of a pattern is obtained by a center-of-gravity calculating means 22 on the basis of the third brightness information, so that the pattern detecting position can be prevented from deviation caused by the background brightness outside the pattern.