To generate a background image from the photographed image of a lecture image or the like photographed by an imaging apparatus while eliminating only objects to be an elimination object such as a figure.

This image processor 1 is provided with a background image storing part 12 for storing an image written on a blackboard, a difference detecting part 13 for detecting a difference between each pixel of the lecture image picked up by the imaging apparatus 100 and each pixel of the image written on the blackboard stored in the background image storing part 12, a determining part 14, a correcting part 15, and a generating part 16 for storing a new background image obtained by substituting the image of an area where persons are present in the lecture images with the image of the area of the image written on the blackboard. The determining part 14 sums up detection results by the detecting part in a block unit, and decides a block in which the number or ratio of pixels determined to have a difference by the detecting part 13 exceeds a prescribed threshold to be a figure block. The correcting part 15 decides blocks adjacent to blocks decided to be the figure block by the determining part 14 to be the figure block in addition to the blocks.