A method for manufacturing an expandable body, a method for manufacturing an expandable body assembly, and a medical device, with which it is possible to suppress breakage of an expandable body during manufacture. The method for manufacturing an expandable body that includes a plurality of struts extending along the axial direction and aligned in the circumferential direction around the axial direction, and has, in the middle in the axial direction, a waist portion having a reduced outer diameter, the method including expanding an inside of a base material having a tubular shape and including the plurality of struts using an inner mold, and restricting an outside of the base material with an outer mold in order to form a bottom having the smallest outer diameter of the waist portion and an upright portion extending radially outward from the bottom.