A developer cartridge comprising: an outside frame comprising: an outside supply opening, and an outside return opening; an inside frame, one of the inside frame and the outside frame being rotatable with respect to another one of the inside frame and the outside frame between an open position and a closed position, the inside frame comprising: a developer chamber for accommodating a developer; an inside supply opening, which is opposed to the outside supply opening when the one of the inside frame and the outside frame is in the open position, for supplying the developer from the developer chamber to an outside of the developer chamber; and an inside return opening, which is opposed to the outside return opening when the one of the inside frame and the outside frame is in the open position, for returning the developer from the outside to an inside of the developer chamber; characterised by a first sealing member that is provided around one of the inside return opening and the outside return opening between the inside frame and the outside frame and is contactable with the inside frame and the outside frame with a first contact pressure; and a second sealing member that is provided around one of the inside supply opening and the outside supply opening between the inside frame and the outside frame and is contactable with the inside frame and the outside frame with a second contact pressure, wherein the second contact pressure is greater than the first contact pressure and wherein a center portion of the inside frame in a longitudinal direction of the inside frame is thicker in the direction perpendicular to the longitudinal direction than end portions of the inside frame in the longitudinal direction, and wherein the inside supply opening is provided in the center portion in the longitudinal direction of the inside frame. an outside frame comprising: an outside supply opening, and an outside return opening; an outside supply opening, and an outside return opening; an inside frame, one of the inside frame and the outside frame being rotatable with respect to another one of the inside frame and the outside frame between an open position and a closed position, the inside frame comprising: a developer chamber for accommodating a developer; an inside supply opening, which is opposed to the outside supply opening when the one of the inside frame and the outside frame is in the open position, for supplying the developer from the developer chamber to an outside of the developer chamber; and an inside return opening, which is opposed to the outside return opening when the one of the inside frame and the outside frame is in the open position, for returning the developer from the outside to an inside of the developer chamber; characterised by a developer chamber for accommodating a developer; an inside supply opening, which is opposed to the outside supply opening when the one of the inside frame and the outside frame is in the open position, for supplying the developer from the developer chamber to an outside of the developer chamber; and an inside return opening, which is opposed to the outside return opening when the one of the inside frame and the outside frame is in the open position, for returning the developer from the outside to an inside of the developer chamber; characterised by a first sealing member that is provided around one of the inside return opening and the outside return opening between the inside frame and the outside frame and is contactable with the inside frame and the outside frame with a first contact pressure; and a second sealing member that is provided around one of the inside supply opening and the outside supply opening between the inside frame and the outside frame and is contactable with the inside frame and the outside frame with a second contact pressure, wherein the second contact pressure is greater than the first contact pressure and wherein a center portion of the inside frame in a longitudinal direction of the inside frame is thicker in the direction perpendicular to the longitudinal direction than end portions of the inside frame in the longitudinal direction, and wherein the inside supply opening is provided in the center portion in the longitudinal direction of the inside frame.