An image processing device (10) includes: a line segment extracting section (14) for generating a line segment image obtained by extracting contour line segments contained in a contour of a subject in a captured image; a candidate quadrilateral calculating section (15) for (i) putting at least one virtual line segment in the line segment image (ii) selecting four line segments from a set containing the at least one virtual line segment and the contour line segments, and (iii) identifying a quadrilateral defined by four straight lines containing respective selected four line segments; and an image correcting section (17) for correcting a distortion in perspective transformation of the captured image based on the quadrilateral identified by the candidate quadrilateral calculating section (15). With the configuration, the distortion of the subject can be corrected without manually adjusting a degree of correction, even in a case where the subject having sides is partially not contained in the captured image or the subject is not a document image.