To secure stability of a sheet at a twisting conveying part for inverting both sides of the sheet during twisting conveyance.

A registration shift amount α of a sheet is measured by a biased twisting position detection part of a twisting conveying device during front-rear inversion of the first sheet P, and is stored in a memory part of a first image forming apparatus (S140). A twisting shift amount α is read from the memory part (S100) when correcting a registration oscillation of a surface of the second sheet. A writing position of an image is moved by the -α of the twisting shift amount (S110). A registration shift amount x1 of the sheet P is measured by a surface registration biased position detection part (S120), a moving amount (-α-x1) for moving the sheet P to a twisting reference position of a twisting conveying part is calculated on the basis of the registration shift amount x1 and the twisting shift amount α, and the sheet is moved in the sheet width direction by the calculated moving amount (S130). Accordingly, a shift of the sheet in a sheet width direction (biased failure) during the twisting conveyance can be prevented.