There is provided a treatment method that is capable of reducing burden on a patient when a medical sheet is indwelled in the body of the patient. A treatment method includes an introduction of introducing a catheter, which has stored a medical sheet (e.g., a myocardial cell sheet), to a heart inside a living body, and an indwelling step of drawing the myocardial cell sheet from the catheter and indwelling the myocardial cell sheet in the heart.