1. An image processing apparatus comprising: an edge pixel extractor circuit for extracting edge pixels from an input image; a line candidate detector circuit for extracting a line candidate from said extracted edge pixels by using Hough transformation; and a line detector circuit for detecting a line from all edge pixels within a predetermined distance from said line candidate extracted from said extracted edge pixels, by using the least square method, wherein when extracting a line candidate under scrutiny by using Hough transformation, said line candidate detector circuit determines a plurality of high-voted line candidates that pass through a higher number of extracted edge pixels than the number of extracted edge pixels passed through by said line candidate under scrutiny, and calculates a number of first edge pixels which are positioned near the line candidate under scrutiny and a number of second edge pixels which are positioned near each of said high-voted line candidates, and when a ratio of the number of second edge pixels to the number of first edge pixels is equal to or greater than a predetermined ratio, said line candidate detector circuit determines that said line candidate under scrutiny and a respective high-voted line candidate are extracted based on same edge pixels, keeps said respective high-voted line candidate as a line candidate, and eliminates said line candidate under scrutiny from the plurality of line candidates.