A cap configured to be attached to a toner supply opening provided in a housing having there inside a space for containing toner, the cap being configured to close the toner supply opening, and the cap comprising: a covering portion configured to cover the toner supply opening; a shaft portion around which a rotary member is to be fitted, the shaft portion configured to rotatably support the rotary member; and a fit-in portion configured to fit into the toner supply opening, wherein the covering portion is configured to cover the toner supply opening when the fit-in portion is in the toner supply opening, wherein the shaft portion extends from the covering portion and away from the housing when the fit-in portion is in the toner supply opening, wherein the covering portion is configured to cover the toner supply opening from an outer side of the housing, and wherein the fit-in portion has a cylindrical shape, and has a base end connected to the covering portion; and a tip end, the tip end being an open end, characterized by a handle portion continuous with the covering portion and extending perpendicularly to the longitudinal axis of the shaft portion, and a breaking portion configured to be broken when the handle portion is moved away from the covering portion and the fit-in portion is removed from the toner supply opening, wherein a thin portion is provided along the boundary between the covering portion and the handle portion, the thin portion being thinner than the covering portion and the handle portion, wherein the thin portion is configured to be broken when the handle portion is pulled, and the portion, in which a slit is to be produced in such a manner as to extend from the thin portion into the covering portion, when the handle portion is pulled, is the breaking portion. a covering portion configured to cover the toner supply opening; a shaft portion around which a rotary member is to be fitted, the shaft portion configured to rotatably support the rotary member; and a fit-in portion configured to fit into the toner supply opening, wherein the covering portion is configured to cover the toner supply opening when the fit-in portion is in the toner supply opening, wherein the shaft portion extends from the covering portion and away from the housing when the fit-in portion is in the toner supply opening, wherein the covering portion is configured to cover the toner supply opening from an outer side of the housing, and wherein the fit-in portion has a cylindrical shape, and has a base end connected to the covering portion; and a tip end, the tip end being an open end, a base end connected to the covering portion; and a tip end, the tip end being an open end, characterized by a handle portion continuous with the covering portion and extending perpendicularly to the longitudinal axis of the shaft portion, and a breaking portion configured to be broken when the handle portion is moved away from the covering portion and the fit-in portion is removed from the toner supply opening, wherein a thin portion is provided along the boundary between the covering portion and the handle portion, the thin portion being thinner than the covering portion and the handle portion, wherein the thin portion is configured to be broken when the handle portion is pulled, and the portion, in which a slit is to be produced in such a manner as to extend from the thin portion into the covering portion, when the handle portion is pulled, is the breaking portion.