1. An image processing apparatus for determining a contour of a particular region in an input image, comprising: an evaluation function obtaining means for performing machine learning in advance for learning a characteristic amount of each pixel in a plurality of sample images, each including a particular region with a known contour, and obtaining an evaluation function capable of evaluating whether or not each pixel is a pixel representing the contour based on the characteristic amount thereof; a point setting means for setting an arbitrary point within the particular region in the input image; a region setting means for setting a discrimination region in the input image, which includes the particular region, with reference to the determined arbitrary point; a characteristic amount obtaining means for obtaining a characteristic amount of each pixel within the determined discrimination region; an evaluation value calculation means for calculating, using the evaluation function, an evaluation value that indicates whether or not each pixel within the discrimination region is a pixel representing the contour based on the obtained characteristic amount thereof; and a contour determination means for determining the contour using the calculated evaluation value, wherein the characteristic amount is combinations of intensity values of a plurality of different pixels within an adjacent region to a target pixel from which the characteristic amount is to be obtained.