Developer container, developing cartridge, process cartridge and image forming apparatus

A developer container comprising:

• a developer accommodating chamber, provided with an opening, for accommodating a developer;
• a sealing member bonded to said developer accommodating container so as to close the opening;
• and
• a rotatable member, to which said sealing member is connected, capable of winding up said seal member by peeling off said sealing member from a bonding portion to said developer container by rotation thereof,
• wherein said rotatable member is constituted so that in a range from an end side to the other end side of a connecting portion to said sealing member with respect to the rotational axis direction, timing from start of winding-up of said sealing member until said sealing member is in a tension state between said rotatable member and the bonding portion is slower with an increasing distance from the end side and so that a speed of an increase in peeling-off amount of said sealing member after said sealing member is in the tension state is slower with an increasing distance from the end side.