A developing unit comprising: a developer cartridge that is configured to accommodate developer and comprises a first opening for enabling communication between an inside of the developer cartridge and an outside of the developer cartridge; a first elastic member that comprises a first through hole and protrudes outward from the developer cartridge, the first through hole surrounding the first opening; a housing comprising: a cartridge housing part for housing the developer cartridge; and a second opening that is provided in a position opposite to the first opening and is formed toward the cartridge housing part; a second elastic member which protrudes from the housing toward the cartridge housing part, and which comprises a second through hole surrounding the second opening; and a shutter that comprises a third opening and is movable between an opening position where the third opening is located at a position between the first opening and the second opening and a closing position where the third opening gets apart from the position between the first opening and the second opening, characterized in that when the shutter is located at the opening position, an upstream end of the third opening in a movement direction of the shutter from the opening position to the closing position is located more upstream than upstream ends of the first through hole and the second through hole in the movement direction, and the second elastic member is contactable with the first elastic member, such that, when the shutter is located at the opening position, the lower circumferential edges of the first elastic member forming the upstream ends of the first through hole are pressed on the lower circumferential edges of the second elastic member forming the upstream ends of the second through hole. a developer cartridge that is configured to accommodate developer and comprises a first opening for enabling communication between an inside of the developer cartridge and an outside of the developer cartridge; a first elastic member that comprises a first through hole and protrudes outward from the developer cartridge, the first through hole surrounding the first opening; a housing comprising: a cartridge housing part for housing the developer cartridge; and a second opening that is provided in a position opposite to the first opening and is formed toward the cartridge housing part; a cartridge housing part for housing the developer cartridge; and a second opening that is provided in a position opposite to the first opening and is formed toward the cartridge housing part; a second elastic member which protrudes from the housing toward the cartridge housing part, and which comprises a second through hole surrounding the second opening; and a shutter that comprises a third opening and is movable between an opening position where the third opening is located at a position between the first opening and the second opening and a closing position where the third opening gets apart from the position between the first opening and the second opening, characterized in that when the shutter is located at the opening position, an upstream end of the third opening in a movement direction of the shutter from the opening position to the closing position is located more upstream than upstream ends of the first through hole and the second through hole in the movement direction, and the second elastic member is contactable with the first elastic member, such that, when the shutter is located at the opening position, the lower circumferential edges of the first elastic member forming the upstream ends of the first through hole are pressed on the lower circumferential edges of the second elastic member forming the upstream ends of the second through hole.