1. An image processing method for determining a contour of a particular region in an input image, comprising the steps of: 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; setting an arbitrary point within the particular region in the input image; setting a discrimination region in the input image, which includes the particular region, with reference to the determined arbitrary point; obtaining a characteristic amount of each pixel within the determined discrimination region; 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 determining the contour using the calculated evaluation value.