1. An image processing device comprising: a binary image generating part that generates binary image data from inputted image data; and a skew angle detecting part that calculates a skew angle of the image data inputted by an input part from the binary image data generated by the binary image generating part, wherein the skew angle detecting part includes: a Hough transform part that executes Hough transform to the binary image data generated by the binary image generating part to generate Hough space data; a frequency calculating part that executes a specific calculation to each of frequencies of data from the Hough space data generated by the Hough transform part, and adds an attained calculation result by each angle to generate first frequency calculation data; and an angle detecting part that calculates an angle from the first frequency calculation data generated by the frequency calculating part.