An image forming device includes: a post processor that performs post processing for folding or forming crease on an adjustment sheet including a reference mark and an adjustment pattern filled in with black, across the adjustment pattern based on a predetermined reference position; an image reader that reads an image of the adjustment sheet under a condition that an orientation of the adjustment sheet is changed according to the post processing type or characteristics of the adjustment sheet which is placed on the platen so that the reference mark is in a predetermined direction; a position identifying unit that identifies a fold line or crease position based on the fold line or crease that appears in the adjustment sheet; and a position adjusting unit that adjusts a fold or crease position of the post processor based on a difference between the fold or crease position and the reference position.