A technique capable of preventing deterioration of image quality resulting from a band-shaped toner image created on a photosensitive surface in order to prevent so-called carrier attraction is provided. This allows the illumination device 1 to efficiently project the white illumination light. The locking means includes a locking portion movably mounted on the first andor second members and actuating means. A protruding bar is integrally provided at a distal end of the partition plate toward the rear port.