A developer cartridge includes an outside frame, an inside frame, a first sealing member and a second sealing member. The outside frame includes an outside supply opening and an outside return opening. The inside frame includes a developer chamber for accommodating a developer, an inside supply opening and an inside return opening. The first sealing member 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. The second sealing member 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 that is greater than the first contact pressure.