A cannulation method includes inserting an endoscope into a duodenum, bringing a distal end section of the endoscope to a position where a duodenal papilla is within a field of view of the endoscope, promoting secretion of pancreatic juice or bile by administering a drainage stimulant, determining an amount of relaxation of the duodenal papilla, and performing cannulation into a biliary duct through the duodenal papilla where the amount of relaxation is greater than a predetermined amount.