A roll sheet cutter mechanism for an image forming apparatus is provided which is adapted to cut a roll sheet 4 paid out of a feed reel 51 between a rotary blade 82 and a stationary blade 81. When the leading edge of the transported roll sheet 4 passes over the rotary blade 82 located at a guiding position, the rotary blade 82 guides the leading edge of the roll sheet 4 in a transportation direction. The rotary blade 82 returns to a cutting stand-by position before the tail edge portion of the roll sheet 4 passes over the rotary blade 82. Therefore, the occurrence of jam of the roll sheet due to the curling thereof can assuredly be prevented. Further, time loss can be eliminated because the rotary blade is adapted to return to the cutting stand-by position.