To provide a circle detecting apparatus which performs high speed processing for detecting a circle form an input image while being reduced in memory use amount.

The circle detecting apparatus (A) includes: an edge image detecting means 3 which applies differential processing to an input image inputted by an imaging means 1 and extracts an edge point that has larger differential intensity than a predetermined threshold; a voting means 4 which votes only for which is within a predetermined angle range centered on an edge direction at the edge point when voting for a (, ) parameter space after applying Hough transformation to the extracted edge point for detecting a straight line; a voted cell extracting means 5 which extracts a voted cell from the (, ) parameter space; a diameter detecting means 6 which detects two approximately parallel sine waves continuing toward a direction from a group of cells extracted by the voted cell extracting means 5 and determines a diameter of a target searched circle according to the direction distance of the two sine waves; and a center position detecting means 7 which determines a center position of the circle according to a figure of an intermediate line which is equal in distance from each of the two sine waves.