PURPOSE: To automatically discriminate whether an inputted medium is positive or negative slide by defining a dark level and a light level with respect to a picture histogram so as to compare with a previously fixed threshold value.

CONSTITUTION: The digital image of the inputted medium being negative or positive slide is received as an input. This digital image is the description of the inputted medium resolved to its color resolving components in the color coordinate system of RGB. The density of the brightest part and the darkest part of an inputted image are calculated by the histogram analysis of these three color resolving. Next, an area of vary high contrast such as one typically generated at an edge part between a perforation and a film boundary and an area of very low contrast such as one typically generated within the perforation and the film boundary are removed. Concerning a histogram generated after removing, quantitative reference is used to discriminate negative or positive.