1. An image processing apparatus for detecting and distinguishing an image pattern from a plurality of image patterns each of which is configured with a circular area and an inner area arranged in the circular area to have a common image pattern portion and a non-common image pattern portion, comprising: a table representing correspondences between the plurality of image patterns and numbers of pixels; a rectangular detection window for detecting an arc segment image included in an image, the arc segment image being a segment of the circular area and being the common image pattern portion of the plurality of image patterns: and a controller capable of performing operations of: receiving image data composed of a plurality of pixels; detecting the arc segment image included in the image based on the received image data, in comparison with a pixel distribution in the rectangular detection window and the correspondences represented by the table; determining plural circular areas having different radiuses, in accordance with the detected arc segment image, each of the plural circular areas being concentric with the circular area; calculating numbers of pixels included in each of the determined plural circular areas; and distinguishing, from the plurality of image patterns, the image pattern included in the received image data on the basis of the calculated numbers of pixels and the correspondences represented by the table.