A developing apparatus comprising: a case having an opening; a developer retainer, which is arranged with a portion of surface being exposed at the opening and is supported by the case so as to be rotatable, for retaining developer at a surface thereof; a blade, which is formed to have a sheet-like shape, is disposed in a longitudinal direction of the developer retainer and has one side end portion in a direction crossing a longitudinal direction thereof that presses a surface of the developer retainer, for regulating a layer thickness of developer to be retained on a surface of the developer retainer; a supporting unit (58,59) for supporting the other side end portion of the blade along a longitudinal direction of the blade; a first fastening member and a second fastening member for fastening the blade to the supporting unit (58,59) respectively at longitudinal end portions of the blade; and a reinforcing portion, which is provided at least between the first fastening member and the second fastening member, for reinforcing the blade, wherein the reinforcing portion is formed at the supporting unit (58,59), wherein the supporting unit (58,59) comprises a first supporting member arranged on one surface of the blade and a second supporting member arranged on the other surface of the blade, the blade is interposed between the first supporting member and the second supporting member, and the reinforcing portion is formed at the first supporting member, wherein the first supporting member comprises a contact portion which extends along a longitudinal direction of the blade and comes in contact with said one surface of the blade, and an extended portion which extends from an end portion opposite to the developer retainer side of the contact portion in a direction crossing said one surface of the blade, and the extended portion functions as the reinforcing portion, characterized in that the first fastening member and the second fastening member respectively run through longitudinal end portions of the contact portion, and the extended portion comprises two end extended parts, which respectively face the first fastening member and the second fastening member in a direction crossing a longitudinal direction of the contact portion and have a first width in an extension direction, and a central extended part, which is provided between the two end extended parts and has a second width larger than the first width in an extension direction. a case having an opening; a developer retainer, which is arranged with a portion of surface being exposed at the opening and is supported by the case so as to be rotatable, for retaining developer at a surface thereof; a blade, which is formed to have a sheet-like shape, is disposed in a longitudinal direction of the developer retainer and has one side end portion in a direction crossing a longitudinal direction thereof that presses a surface of the developer retainer, for regulating a layer thickness of developer to be retained on a surface of the developer retainer; a supporting unit (58,59) for supporting the other side end portion of the blade along a longitudinal direction of the blade; a first fastening member and a second fastening member for fastening the blade to the supporting unit (58,59) respectively at longitudinal end portions of the blade; and a reinforcing portion, which is provided at least between the first fastening member and the second fastening member, for reinforcing the blade, wherein the reinforcing portion is formed at the supporting unit (58,59), wherein the supporting unit (58,59) comprises a first supporting member arranged on one surface of the blade and a second supporting member arranged on the other surface of the blade, the blade is interposed between the first supporting member and the second supporting member, and the reinforcing portion is formed at the first supporting member, wherein the first supporting member comprises a contact portion which extends along a longitudinal direction of the blade and comes in contact with said one surface of the blade, and an extended portion which extends from an end portion opposite to the developer retainer side of the contact portion in a direction crossing said one surface of the blade, and the extended portion functions as the reinforcing portion, characterized in that the first fastening member and the second fastening member respectively run through longitudinal end portions of the contact portion, and the extended portion comprises two end extended parts, which respectively face the first fastening member and the second fastening member in a direction crossing a longitudinal direction of the contact portion and have a first width in an extension direction, and a central extended part, which is provided between the two end extended parts and has a second width larger than the first width in an extension direction.