A developing device comprising: a developer bearer to transport by rotation developer carried on a surface thereof to a development range to supply developer to a latent image bearer; a development casing in which developer conveyance channels (53a,54a) are formed, the developer conveyance channels (53a,54a) facing the developer bearer through communicating areas (53b,54b) inside the development casing; a developer supply member disposed in the developer conveyance channels (53a,54a) to transport developer in an axial direction of the developer bearer and supply developer to the developer bearer; a developer regulator disposed facing a lower portion of the developer bearer to adjust an amount of developer carried on the developer bearer; and a sheet member removably installed in the development casing to seal the communicating areas (53b,54b), wherein the sheet member passes through a gap between the developer regulator and the developer bearer and an opening formed in the development casing, a first end of the sheet member is retained downstream from the gap in a direction of rotation of the developer bearer, the sheet member is fixed to the development casing at a position across the communicating areas (53b,54b) from the first end thereof with a second end of the sheet member opposite the first end positioned outside the development casing, a developer collecting member to transport axially developer received from the developer bearer at a position downstream from the development range in the direction of rotation of the developer bearer; and a partition to divide the developer conveyance channels (53a,54a) into a supply channel in which the developer supply member is disposed and a collecting channel in which the developer collecting member is disposed, wherein the communicating areas (53b,54b) include a first communicating opening where the developer bearer faces the supply channel and a second communicating opening where the developer bearer faces the collecting channel, and the sheet member seals both the first communicating opening and the second communicating opening, characterized in that a guide face to guide the sheet member is provided at either end of the development casing in the axial direction of the developer bearer, an end face of the partition facing the developer bearer is continuous with the guide faces at the respective ends in the axial direction, in the direction of rotation of the developer bearer, the guide face extends from the end face of the partition to the developer regulator, and a distance between the guide face and the surface of the developer bearer is identical to a distance between the end face of the partition and the surface of the developer bearer. a developer bearer to transport by rotation developer carried on a surface thereof to a development range to supply developer to a latent image bearer; a development casing in which developer conveyance channels (53a,54a) are formed, the developer conveyance channels (53a,54a) facing the developer bearer through communicating areas (53b,54b) inside the development casing; a developer supply member disposed in the developer conveyance channels (53a,54a) to transport developer in an axial direction of the developer bearer and supply developer to the developer bearer; a developer regulator disposed facing a lower portion of the developer bearer to adjust an amount of developer carried on the developer bearer; and a sheet member removably installed in the development casing to seal the communicating areas (53b,54b), wherein the sheet member passes through a gap between the developer regulator and the developer bearer and an opening formed in the development casing, a first end of the sheet member is retained downstream from the gap in a direction of rotation of the developer bearer, the sheet member is fixed to the development casing at a position across the communicating areas (53b,54b) from the first end thereof with a second end of the sheet member opposite the first end positioned outside the development casing, a developer collecting member to transport axially developer received from the developer bearer at a position downstream from the development range in the direction of rotation of the developer bearer; and a partition to divide the developer conveyance channels (53a,54a) into a supply channel in which the developer supply member is disposed and a collecting channel in which the developer collecting member is disposed, wherein the communicating areas (53b,54b) include a first communicating opening where the developer bearer faces the supply channel and a second communicating opening where the developer bearer faces the collecting channel, and the sheet member seals both the first communicating opening and the second communicating opening, characterized in that a guide face to guide the sheet member is provided at either end of the development casing in the axial direction of the developer bearer, an end face of the partition facing the developer bearer is continuous with the guide faces at the respective ends in the axial direction, in the direction of rotation of the developer bearer, the guide face extends from the end face of the partition to the developer regulator, and a distance between the guide face and the surface of the developer bearer is identical to a distance between the end face of the partition and the surface of the developer bearer.