Developing device, image forming unit and image forming apparatus

A developing device comprising:

• a developing device main body, and
• a developer cartridge detachably mounted to said developing device main body and storing a developer, said developer cartridge comprising:
• a case member that stores said developer;
• a supplying opening provided on said case member, and
• a first shutter rotatably provided in said case member to open and close said supplying opening, said first shutter has a first lock member that engages said case member so as to lock a movement of said first shutter, said developing device main body comprising:
• a housing that stores said developer supplied by said developer cartridge;
• a receiving opening provided on said housing, and
• a second shutter provided on said developing device main body so as to be slidable along an outer circumferential surface of said developer cartridge to open and close said receiving opening, wherein said first shutter and said second shutter are configured to move in conjunction with each other, wherein said first shutter or said second shutter has an operating portion which is operated by a user to move said first shutter and said second shutter to open and close said supplying opening and said receiving opening, wherein said housing of said developing device main body has a second lock member that locks a movement of said second shutter, and wherein, when said developer cartridge is mounted to said developing device main body, said first lock member is pressed by a part of said housing and allows said first shutter to move, and said first engaging portion engages said second engaging portion and also presses said second lock member so that said second lock member allows said second shutter to move.