Developing unit and image forming apparatus

A developing unit comprising:

• a developer retainer that includes a magnet member and a retaining member, the magnet member having magnetic poles, the retaining member being supported on an outer circumference of the magnet member and retaining a developer magnetized and sucked on a surface of the retaining member by a magnetic force of the magnet member;
• a developing vessel that receives the developer internally and that includes a retainer receiving portion, a first agitation portion and a second agitation portion, the retainer receiving portion receiving the developer retainer, the first agitation portion being adjacent to the retainer receiving portion, the second agitation portion being adjacent to the first agitation portion;
• a first conveyance member that is received in the first agitation portion and that conveys the developer in the first agitation portion in a first conveyance direction;
• a second conveyance member that is received in the second agitation portion and that conveys the developer in the second agitation portion in a second conveyance direction which is reverse to the first conveyance direction;
• a layer thickness limiting member that is provided in opposition to the developer retainer to limit a layer thickness of the developer retained on the surface of the retaining member, that is disposed between the second conveyance member and the developer retainer and that is made of a magnetic material;
• and
• a reduction portion that is disposed on an entrance side of the layer thickness limiting member and in a space between the layer thickness limiting member and the second agitation portion, the reduction portion reducing entrance of the developer on an upstream side of the layer thickness limiting member into the space.