A unit position adjustment method for adjusting, relative to an image forming device main body, a fixed position of a surface mobile body unit comprising a surface mobile body for supporting/transporting a toner image, or a surface mobile body for transporting a recording medium, comprising the step of adjusting, by means of a unit position adjustment member, the position of a unit support member relative to a structure that forms a framework of the image forming device main body, the unit support member being a member other than the structure and fixing the surface mobile body unit to the structure, to adjust thereby the fixed position of the surface mobile body unit relative to the image forming device main body.