A high-precision image control method that can be conducted frequently without time or labor. When a sheet-passing operation starts, maximum exposure is performed at present settings in a predetermined non-image forming area and potential is detected by a potential sensor. Detected potential is compared with V.sub.H at a first control to judge whether a difference of 10 V or more exists. For a difference of 10 V or more, the signal is made responsive to laser output for achieving V.sub.H set at the first control. When the last image forming for that job is performed, potential returns to the potential set at the first control, and control ends. When the last image forming is completed in a consecutive job during short-term variability of V.sub.H, V.sub.H is restored to its original potential before the next job, whereby settings obtained with the first control are restored.