An image processing apparatus includes an edge pixel extractor for extracting an edge pixel from an input image, an end pixel extractor for extracting an end pixel in the input image, an edge pixel addition module for adding the end pixel extracted by the end pixel extractor as an edge pixel based on a pixel value of a pixel located within a first range from the end pixel, a straight line extractor for extracting straight lines from the edge pixel extracted by the edge pixel extractor and the end pixel added as an edge pixel by the edge pixel addition module, and a rectangle region detector for detecting a rectangle region from the extracted straight lines.