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 which includes a document, a grouping unit for forming mutually adjoining edge pixels in the extracted edge pixels into a group, and an approximated line detector for detecting a line which connects edge pixels which are positioned at the two ends in a horizontal direction and vertical direction among edge pixels which are included in the group as a line which approximates an end of the document.