When a data control portion receives a request for change of a pixel density, a cycle used as a reference cycle for detecting abnormality in a laser beam is changed corresponding to the pixel density. While a rotating speed of a polygon mirror for changing the density of picture element is not constant, emission of the laser beam is forbidden and detection as to abnormality in the laser beam is not effected. A mark for detection of an image density is formed on a photoreceptor where the laser beam is scanned, and outside an area where the image is to be formed. The formation of the detection mark is forbidden while the pixel density is being changed. On the other hand, when the detection mark is being formed, output of an instruction for changing the pixel density is deferred. In addition, emission of the laser beam is forbidden while the pixel density is being changed.