1. An image processing device comprising: step calculating means for calculating a step of each pixel in an image, the step being obtained by subtracting, from an absolute value of a difference in pixel values between the pixel and one adjacent reference pixel of the pixel, an average value of absolute values of differences in pixel values between any two in a neighborhood among the pixel and reference pixels of the pixel except for a neighborhood between the pixel and the one adjacent reference pixel of the pixel, wherein the reference pixels or the pixel are all horizontal pixels of the pixel of all vertical pixels of the pixel, wherein the step calculating means further includes storing means for storing the step of the pixel, and when the step of the pixel of interest is a largest one among the step of the pixel of interest and the steps of the reference pixels of the pixel of interest stored by the storing means, the step calculating means sets the step of the pixel of interest as a block boundary step; classifying means for classifying the pixels into classes for areas of the block boundary steps; boundary ratio calculating means for calculating, as a boundary ratio, a ratio of the number of pixels at a block boundary for each of the classes to the number of pixels in the image for the each of the classes; and block noise strength determining means for determining, as a block noise strength of the image, the block boundary step that is larger than a predetermined threshold and that is at a class having a largest value.