Patent ID: 8582994

Claim:
An image forming system comprising: a first image forming apparatus to form an image on a first side of a recording medium, the first image forming apparatus comprising a mark forming device to form a mark having a first mark portion and a second mark portion on the recording medium, the first mark portion having a length variable in a first direction along a conveyance direction of the recording medium based on a position of the mark relative to a second direction perpendicular to the conveyance direction, the second mark portion having a line form parallel to one side of the first mark portion except for a side in parallel to the first direction, with a predetermined gap between the first mark portion and the second mark portion; a reversing device to reverse the recording medium; and a second image forming apparatus to form an image on a second side that is the obverse of the first side, the second image forming apparatus comprising a mark detector to detect the mark formed by the mark forming device of the first image forming apparatus at a predetermined position along a conveyance path of the recording medium while the mark passes through the predetermined position and output a first passing-through time, representing a time taken for a portion of the predetermined gap between the first mark portion and the second mark portion to pass through the mark detection position, and a second passing-through time, representing a time taken for the first mark portion to pass through the predetermined position, and a calculator to calculate an expansion and contraction ratio of the recording medium in the first direction and an expansion and contraction ratio of the recording medium in the second direction based on the first passing-through time and the second passing-through time output by the mark detector, wherein the first image forming apparatus further comprises a density detector to detect a density of the mark and the second image forming apparatus further comprises a sensitivity adjustment device to adjust a sensitivity of the mark detector based on the density of the mark detected by the density detector.