The present invention provides an image forming device capable of forming high-quality images and affording high-precision rotating shaft parallelism between surface mobile bodies, and a unit position adjustment method for adjusting the position of a surface mobile body unit relative to the main body of the image forming device. The image forming device includes a front fixing guide plate as a unit support member for fixing, to a front plate of a structure, a fixing unit serving as a surface mobile body unit which includes a fixing roller being a surface mobile body, and an upper portion eccentric cam and a horizontal portion eccentric cam serving as a unit position adjustment member for adjusting the position of the front fixing guide plate relative to the front plate.