1. An image processing system comprising: an image reception unit that receives an image including an additional entry portion; a first additional entry portion extracting unit that extracts the additional entry portion from the image, which is received by the image reception unit, by using a first threshold; a threshold calculating unit that calculates a second threshold based on a feature in an area alongside the additional entry portion extracted by the first additional entry portion extracting unit; a second additional entry portion extracting unit that extracts the additional entry portion from the image, which is received by the image reception unit, by using the second threshold calculated by the threshold calculating unit; a concealment area specifying unit that specifies an area to be concealed in the image based on the additional entry portion extracted by the second additional entry portion extracting unit; and an image concealment unit that conceals the area specified by the concealment area specifying unit in the image, wherein the threshold calculating unit uses one of pixel density and line segment amount in the area as the feature in the area, wherein the line segment amount used by the threshold calculating unit is the amount of a line segment along a direction of a shorter side in a case where the length of a longitudinal side of the additional entry portion extracted by the first additional entry portion extracting unit is compared with the length of a lateral side thereof.