1. An image processing system which performs image processing based on a difference image, the difference image being a difference image between a first calibration image and a second calibration image and being configured of: a center block area positioned in a central part of the difference image; a peripheral block area positioned on a periphery of the center block area; and a background area which is an area other than the center block area and the peripheral block area, the image processing system comprising: image projection means for projecting the first and second calibration images at different timings onto a projection target; sensing means for sensing the projected first calibration image at a low resolution which is less than or equal to a predetermined resolution and generating first sensing information, sensing the first calibration image at a high resolution which is greater than or equal to the low resolution and generating third sensing information, and sensing the second calibration image at the high resolution and generating second sensing information; projection area detection means for generating projection area information relating to a position of a projection area in a sensing area of the sensing means, based on the first and second sensing information; projection-target area detection means for generating projection-target area information relating to a position of a projection-target area corresponding to the projection target in the sensing area of the sensing means, based on the first, second, and third sensing information; and image distortion correction means for correcting distortion of an image to be projected by the image projection means, based on the projection area information and the projection-target area information, wherein at least an outer peripheral portion of the second calibration image has a brightness index value of zero, wherein the projection area detection means comprises: difference image generation means for generating the difference image, based on the first and second sensing information; center reference position detection means for detecting a plurality of center reference positions of the center block area in the sensing area of the sensing means, based on the difference image; peripheral reference position detection means for detecting a plurality of peripheral reference positions of the peripheral block area in the sensing area, based on the center reference positions; and projection area information generation means for generating the projection area information, based on the center reference positions and the peripheral reference positions, wherein the projection-target area detection means comprises: search range determination means for setting a search range for edge detection in an outer portion of the center block area; edge detection means for performing edge detection based on the first sensing information and the search range and generating first edge detection information, and also performing edge detection based on the third sensing information and generating third edge detection information; and projection-target area information generation means for generating provisional detection information by provisionally detecting the projection-target area based on the first edge detection information, and also generating the projection-target area information based on the third edge detection information, and wherein the edge detection means generates the third edge detection information by performing edge detection on a pixel group around a boundary of the provisionally detected projection-target area, based on the provisional detection information.