1. An image density checking chart for checking whether a density of a test image on a sheet outputted by a full-color image forming apparatus is within a predetermined density range or not by an operator, said chart comprising: an exposing portion configured to expose the test image when the chart is overlaid on the sheet; a target density image configured to be compared to the test image to determine whether the density of the test image is a target density level or not; an upper limit density image configured to be compared to the test image to determine whether the density of the test image is not higher than an upper limit density level of the predetermined density range or not; and a lower limit density image configured to be compared to the test image to determine whether the density of the test image is not lower than a lower limit density level of the predetermined density range or not, wherein only said target density image, said upper limit density image and said lower limit density image are disposed around a periphery of said exposing portion, wherein said exposing portion is a general rectangular configuration, and wherein only said target density image, said upper limit density image, and said lower limit density image are disposed at three sides of said exposing portion, wherein said target density image, said upper limit density image, and said lower limit density image have substantially the same size with said exposing portion, and wherein each of said target density image, said upper limit density image and said lower limit density image have one density level.