A developing device (3a to 3d) comprising: a developing roller that is configured to be disposed to oppose an image carrier (1a to 1d) which is disposed outside and adjacent to the developing device (3a to 3d), wherein an electrostatic latent image is formed on the image carrier (1a to 1d) and wherein the developing roller supplies a two-component developer containing a magnetic carrier and a toner to the image carrier (1a 1d) at a region that opposes the image carrier (1a to 1d); a toner supplying roller that is disposed to oppose the developing roller and supplies the toner of the two-component developer to the developing roller at a region that opposes the developing roller; a regulation blade that is disposed to oppose the toner supplying roller over a predetermined gap; a casing that houses the developing roller, the toner supplying roller, the regulation blade and an inner wall portion which opposes the developing roller between the regulation blade and the image carrier (1a to 1d); characterized in that the casing comprises: a flexible film member that is formed of a flexible material, disposed away from an upper surface of the inner wall portion by a predetermined gap and vibratile in a direction to approach or leave the inner wall portion; a bias member that is connected to at least one end in a longitudinal direction of the film member and gives tension to the film member; and a protrusion that is disposed on a gear composing a group of drive gears of the developing roller or the toner supplying roller to protrude and overlap an end edge of the film member and intermittently contacts the end edge of the film member thanks to rotation of the gear to vibrate the film member. a developing roller that is configured to be disposed to oppose an image carrier (1a to 1d) which is disposed outside and adjacent to the developing device (3a to 3d), wherein an electrostatic latent image is formed on the image carrier (1a to 1d) and wherein the developing roller supplies a two-component developer containing a magnetic carrier and a toner to the image carrier (1a 1d) at a region that opposes the image carrier (1a to 1d); a toner supplying roller that is disposed to oppose the developing roller and supplies the toner of the two-component developer to the developing roller at a region that opposes the developing roller; a regulation blade that is disposed to oppose the toner supplying roller over a predetermined gap; a casing that houses the developing roller, the toner supplying roller, the regulation blade and an inner wall portion which opposes the developing roller between the regulation blade and the image carrier (1a to 1d); characterized in that the casing comprises: a flexible film member that is formed of a flexible material, disposed away from an upper surface of the inner wall portion by a predetermined gap and vibratile in a direction to approach or leave the inner wall portion; a bias member that is connected to at least one end in a longitudinal direction of the film member and gives tension to the film member; and a protrusion that is disposed on a gear composing a group of drive gears of the developing roller or the toner supplying roller to protrude and overlap an end edge of the film member and intermittently contacts the end edge of the film member thanks to rotation of the gear to vibrate the film member. a flexible film member that is formed of a flexible material, disposed away from an upper surface of the inner wall portion by a predetermined gap and vibratile in a direction to approach or leave the inner wall portion; a bias member that is connected to at least one end in a longitudinal direction of the film member and gives tension to the film member; and a protrusion that is disposed on a gear composing a group of drive gears of the developing roller or the toner supplying roller to protrude and overlap an end edge of the film member and intermittently contacts the end edge of the film member thanks to rotation of the gear to vibrate the film member.