1. An image processing device comprising: a processor; a projector configured to be controlled by the processor; and a memory which stores a plurality of instructions, which when executed by the processor, cause the processor to execute: acquiring an image, including a first object having a character or a figure and a second object, captured by an image sensor; computing, from the image, flatness information on flatness of a projection plane of the first object on which a superimposed image is to be displayed by the projector, wherein the superimposed image is relevant to the character or the figure and includes additional information associated with the first object, wherein the flatness information is defined from three-dimensional rectangular coordinates of the first object computed from the image; extracting display candidate regions based on a predetermined feature quantity with respect to the first object and the three-dimensional rectangular coordinates of the first object computed from the image; defining plane rectangular coordinates serving as a display position, which is a partial part of the first object for displaying the superimposed image by the projector on the basis of the flatness information and a dispersion of the plurality of the display candidate regions relative to a reference point; and projecting the superimposed image on the partial part of the first object.