Abstract: The present disclosure relates to a method for providing information necessary for assessing severity of coronary artery stenosis, the method includes: administering a contrast agent to a coronary artery; capturing an angiographic image; setting a suspected stenosis area where stenosis is suspected and a proximal area where blood passes before the suspected stenosis area within region of captured image for observing the coronary artery stenosis based on the captured angiographic image; fixing position of the coronary artery where the contrast agent is administered by image processing; and deriving a blood flow velocity ratio, which is a relative ratio of blood flow in the proximal area and the suspected stenosis area, based on time when brightness changes in the proximal area and the suspected stenosis area of the captured image.