A drug is applied to a wall portion of a body cavity through a drug administration sheath including a first lumen that supplies a drug and a second lumen that is provided with at least one balloon. The drug administration sheath is inserted through an ostium of the body cavity until a distal end of the sheath reaches a deep part of the body cavity. The drug supplied through the first lumen is applied to a mucosa in the body cavity, utilizing inflation and movement of the balloon provided in the second lumen inside the body cavity so as to spread the drug along the mucosa.
