A developing device frame unit for supporting a regulating member for regulating a layer thickness of a developer on a developer carrying member, includes a developing device frame having a seal forming portion; one end sealing member which is provided at one longitudinal end of the developing device frame and which is contactable to a surface of the developer carrying member to prevent the developer from leaking in an axial direction of the developer carrying member when the developer carrying member is mounted; the other end sealing member which is provided at the other longitudinal end of the developing device frame and which is contactable to the surface of the developer carrying member to prevent the developer from leaking in an axial direction of the developer carrying member when the developer carrying member is mounted; a blade sealing member for sealing between the regulating member and the developing device frame to prevent the developer from leaking when the regulating member is mounted , wherein the blade sealing member is an elastomer resin material which is injection-molded with a metal mold and which is provided in the seal forming portion where the one end sealing member and the other end sealing member are provided, the blade sealing member connecting the one end sealing member and the other end sealing member with each other; and a protrusion provided by squeezed-out portion of the elastomer resin material, wherein the squeezed-out portion is provided by injecting, into a space defined by the metal mold, the seal forming portion, the one end sealing member and the other end sealing member, a volume of the elastomer resin material larger than a volume of the space.