A cartridge includes: a casing having a wall defining a developer accommodating portion inside the casing, a filling port being formed through the wall in fluid communication with the developer accommodating portion; a cylindrical wall portion formed on the wall at a circumferential edge of the filling port and protruding from the wall outside the casing, the cylindrical wall portion having an inner peripheral surface, an outer peripheral surface, and an end surface positioned between the inner peripheral surface and the outer peripheral surface; a closure member which is fitted in the cylindrical wall portion, while being in contact with the inner peripheral surface of the cylindrical wall portion, to close the filling port; and a protruding portion that is provided on at least one of the end surface and the closure member at its contact portion between the end surface and the closure member and that protrudes toward the other one of the end surface and the closure member.