There are provided an image processing apparatus, line detection method and a computer-readable, non-transitory medium that can precisely detect boundaries of a document from a read image. The image processing apparatus includes an edge pixel extractor for extracting edge pixels from an input image, a line candidate detector for extracting a line candidate from the extracted edge pixels by using Hough transformation, and a line detector for detecting a line from edge pixels within a predetermined distance from the line candidate by using the least square method.