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. A defect test is implemented for generating redundant information. Also included is an output waveguide having a Bragg grating adapted for coupling the wavelength i into the output waveguide in a coupling zone. If the corrected JCL ends abnormally due to an execution error, the method then automatically determines if the execution error is recoverable.