An image forming device capable of implementing a change of resolution and capable of forming an image with few linear flaws in a main scanning direction. A recording material is slantedly wound onto a rotary drum at an angle such that, if recording at a predetermined resolution, main scanning lines that are recorded are parallel with both ends of the recording material along a direction of winding onto the drum. Whenever a main scanning line being recorded on the recording material is displaced, relative to a main scanning line that would be recorded at the predetermined resolution, by precisely an amount corresponding to a pre-specified number of pixels, image data is shifted in a direction opposite to the direction of the displacement by the amount corresponding to the pre-specified number of pixels.