1. A projector for displaying an image on a screen comprising: a light source for emitting light; an image forming panel for forming, in an image forming area of a panel surface, an effective panel image for modulating light that is emitted from the light source into an effective image light that represents an image; an image capturing unit configured to generate a captured image by capturing at least a specific projection area wherein the image light , corresponding to a specific area within the image forming area, is projected; an outside perimeter line detecting unit configured to calculate one or more edges of outside perimeter lines of the specific projection area and one or more edges of the screen, through analyzing the captured image and detecting plural points on the outside perimeter lines of the specific projection area and of the screen within the captured image; a post-correction image forming area calculating unit configured to calculate, based on detection results by the outside perimeter line detecting unit, post-correction image forming area that is a part of the image forming area ; and a keystone correcting unit configured to correct trapezoidal distortion of an image that is displayed on the screen through forming of the effective panel image in the post-correction image forming area within the image forming area , wherein when only three or less edges among four edges of the outside perimeter lines of the screen have been detected by the outside perimeter line detecting unit, the post-correction image forming area calculating unit calculates the post-correction image forming area based on a position and a slope of the detected edge within the captured image.