1. An image processing method comprising: generating a subtraction image obtained by performing subtraction processing between a mask radiation image before an inflow of contrast medium and a live radiation image after the inflow of contrast medium; setting an emphasis degree for enhancing of the subtraction image; determining, for each pixel included in the subtraction image, existence or absence of an edge in the live radiation image based on a pixel value of a pixel of the live radiation image at a position corresponding to a position of the pixel in the subtraction image, and determining a correction coefficient to correct the emphasis degree in accordance with the existence or absence of the edge; and enhancing pixel value for each of a plurality of pixels included in the subtraction image based on the emphasis degree and the correction coefficient.