PURPOSE: To secure the same level of quantity between the positive and negative pictures with a laser output device, by changing just the beam diameter with use of the same bit data.

CONSTITUTION: An optimum beam diameter is obtained by performing a switching action between the positive and negative beam diameter conversion lenses 4a and 4b. In such a constitution of an optical system, the beam diameter is set approximately equal to the scanning line density on a photosensitive material by a beam expander 5 before the lens 4 is put into an optical path. Thus an optimum beam diameter is obtaind by putting the lens 4 into the optical path.