In an image processor for detecting a circular pattern in an image, input image
data is binarized to provide bi-level image data, and pixels having a predetermined
value is counted in a block of a polygon having n vertices in the bi-level image
data, wherein n denotes a natural number equal to or larger than eight. Then, it
is decided, based on a number of the pixels having the predetermined value counted
by the counter, whether the circular pattern is detected in the image or not. A
detection window is used to detect the specified pattern. The detection window
is moved successively by a predetermined number of pixels, in a direction from
a side towards the center. The moving distance and direction are controlled based
on the result of scan.