An image forming device includes: a fold unit that folds an adjustment sheet in which a first adjustment pattern including at least one line segment, and a second adjustment pattern filled in with black are formed, based on a predetermined reference fold position to across the first and second adjustment patterns; an image reader that reads an image of the first adjustment pattern and an image of the second adjustment pattern of the adjustment sheet with the adjustment sheet unfolded; a position identifying unit that identifies a position of a fold line based on distortion of the line segment on the image of the first adjustment pattern, or a fold line on the image of the second adjustment pattern; and a position adjusting unit that adjusts a fold position of the fold unit based on a difference between the position of the fold line and the reference fold position.