A developer container, comprising: a cap configured to control a flow of developer and comprising a hole configured to be detachably engaged with a nozzle connecting to an image forming apparatus; and a developer case configured to contain the developer and comprising an outlet configured to be connected to the hole, the developer case allowing the developer to flow through the outlet in a direction different from the flow of developer through the hole, the developer case comprising: a bag including a flexibly foldable material and having at least one sheet member attached on a surface thereof, wherein the bag is configured to decrease in volume when an inner pressure thereof decreases and wherein the at least one sheet member is configured to reinforce the surface of the bag; wherein: the bag has an upper portion with first surfaces with a fold (f) and second surfaces without a fold, the second surfaces disposed facing to each other, and a lower portion tapered towards an opening of the bag, the at least one sheet member adheres to a second surface at the upper portion; characterized by the at least one sheet member having a laterally bow-shaped surface extending outwardly from the surface of the bag. a cap configured to control a flow of developer and comprising a hole configured to be detachably engaged with a nozzle connecting to an image forming apparatus; and a developer case configured to contain the developer and comprising an outlet configured to be connected to the hole, the developer case allowing the developer to flow through the outlet in a direction different from the flow of developer through the hole, the developer case comprising: a bag including a flexibly foldable material and having at least one sheet member attached on a surface thereof, wherein the bag is configured to decrease in volume when an inner pressure thereof decreases and wherein the at least one sheet member is configured to reinforce the surface of the bag; wherein: the bag has an upper portion with first surfaces with a fold (f) and second surfaces without a fold, the second surfaces disposed facing to each other, and a lower portion tapered towards an opening of the bag, the at least one sheet member adheres to a second surface at the upper portion; characterized by the at least one sheet member having a laterally bow-shaped surface extending outwardly from the surface of the bag. a bag including a flexibly foldable material and having at least one sheet member attached on a surface thereof, wherein the bag is configured to decrease in volume when an inner pressure thereof decreases and wherein the at least one sheet member is configured to reinforce the surface of the bag; wherein: the bag has an upper portion with first surfaces with a fold (f) and second surfaces without a fold, the second surfaces disposed facing to each other, and a lower portion tapered towards an opening of the bag, the at least one sheet member adheres to a second surface at the upper portion; characterized by the at least one sheet member having a laterally bow-shaped surface extending outwardly from the surface of the bag. the bag has an upper portion with first surfaces with a fold (f) and second surfaces without a fold, the second surfaces disposed facing to each other, and a lower portion tapered towards an opening of the bag, the at least one sheet member adheres to a second surface at the upper portion; characterized by the at least one sheet member having a laterally bow-shaped surface extending outwardly from the surface of the bag.