An image forming apparatus includes a casing 2 and a process cartridge 20, which is mounted removably in the casing. A powder retaining box 53 is supported pivotally by the bottom of the process cartridge 20, and supports a pinch roller 52 for contacting with a sheet of paper 3 to collect paper powder from it. The retaining box 53 has an opening through which the collected powder enters it. The pinch roller 52 substantially closes the box opening, and prevents the paper powder in the retaining box 53 from scattering out when the process cartridge 20 is dismounted from the casing 2. If the retaining box 53 were fixed to the process cartridge 20, and the pinch roller 52 moved relative to the retaining box when the process cartridge is dismounted, paper powder might scatter out of the retaining box.