With a minimal process load, to determine level of the image of an image data file, for image data files of a format that saves images in the form of coefficients of discrete frequency components. Image data that includes a plurality of coefficients representing the pattern of change of color of the image are acquired (Step S10). Multiple sets of basic pattern data that respectively include a plurality of basic coefficients representing basic patterns of color change are prepared. Then, based on a comparison of the two, one set of selected basic pattern data is selected from among the multiple sets of basic pattern data (Step S20). Subsequently, the blur level of the image of the image data is calculated, based on the basic blur level associated with the selected basic pattern data (S30-S70).