A developer unit for an image forming apparatus to form an image on a recording sheet, comprising: a developer device having a developer agent carrier, which carries a developer agent on a surface thereof and which is adapted to supply the developer agent to a latent image carrier, and a developer agent supplier, which supplies the developer agent to the developer agent carrier; and a developer agent container, which contains the developer agent to be supplied to the developer device, wherein the developer agent container is formed to have a fitting wall, which is curved inward at a position to be adjacent to the developer device; wherein the fitting wall is formed to have a feeding opening, through which the developer agent in the developer agent container is supplied to the developer device, and a collecting opening, through which the developer agent in the developer device is retrieved; and wherein the developer unit is further provided with: a first conveyer, which is rotated about a rotation axis to sweep an inner surface of the developer agent container and convey the developer agent toward the feeding opening; and a second conveyer, which is arranged along the developer agent supplier and conveys the developer agent toward the collecting opening, characterized in that the fitting wall is curved inward in an arc in cross-section to center around a reference line, which extends in parallel with a rotation axis of the first conveyer, and the developer agent container is arranged in a lower position with respect to the developer device such that the developer agent container is under both the developer agent supplier and the second conveyor when the developer agent container is installed in the image forming apparatus. a developer device having a developer agent carrier, which carries a developer agent on a surface thereof and which is adapted to supply the developer agent to a latent image carrier, and a developer agent supplier, which supplies the developer agent to the developer agent carrier; and a developer agent container, which contains the developer agent to be supplied to the developer device, wherein the developer agent container is formed to have a fitting wall, which is curved inward at a position to be adjacent to the developer device; wherein the fitting wall is formed to have a feeding opening, through which the developer agent in the developer agent container is supplied to the developer device, and a collecting opening, through which the developer agent in the developer device is retrieved; and wherein the developer unit is further provided with: a first conveyer, which is rotated about a rotation axis to sweep an inner surface of the developer agent container and convey the developer agent toward the feeding opening; and a second conveyer, which is arranged along the developer agent supplier and conveys the developer agent toward the collecting opening, a first conveyer, which is rotated about a rotation axis to sweep an inner surface of the developer agent container and convey the developer agent toward the feeding opening; and a second conveyer, which is arranged along the developer agent supplier and conveys the developer agent toward the collecting opening, characterized in that the fitting wall is curved inward in an arc in cross-section to center around a reference line, which extends in parallel with a rotation axis of the first conveyer, and the developer agent container is arranged in a lower position with respect to the developer device such that the developer agent container is under both the developer agent supplier and the second conveyor when the developer agent container is installed in the image forming apparatus.