1. An image processing device determining a depth of a target object in real space by performing image processing, the image processing device comprising: a generator generating a histogram for a designated colour in frame image data obtained by filming the real space, the histogram indicating each of a plurality of designated colour pixel counts at each of a plurality of coordinates in a reference axis of a screen; a smoother performing smoothing of a generated histogram; and a depth determiner selecting one among the designated colour pixel counts indicated in a smoothed histogram, a selected count being associated with a specific set of coordinates, and determining the depth of the target object using the selected count.