1. An image processing device comprising: an imaging section equipped with an image sensor; a first binarization section which performs processing to binarize an actual image captured in the image sensor in accordance with an adaptive thresholding; a first detecting section which detects an augmented reality marker from within the actual image binarized by the first binarization section; a threshold determining section which determines a binarization threshold based on the augmented reality marker detected by the first detecting section; a second binarization section which performs processing to binarize the actual image captured in the image sensor in accordance with a fixed threshold binarization method using the binarization threshold, after the binarization threshold is determined by the threshold determining section; and a second detecting section which detects the augmented reality marker based on the actual image binarized by the second binarization section.