To suppress the occurrence of irregular density which may appear by long continuation of an operation stop state, and to stably form a toner image of fine quality.

The operation thereafter is changed in accordance with the length of operation stop time ts from the end of image formation. When an image signal is inputted from an external device before the arrival of the operation stop time ts at 1st downtime t1, image formation corresponding to the image signal is immediately performed (Fig.(a)). When the image signal is inputted in the operation stop time of the 1st downtime t1 and more and less than 2nd downtime t2, optimizing processing including patch image formation is performed, and after optimizing a density control factor, image formation corresponding to the image signal is performed (Fig.(b)).