luminous power data which are already stored in the read only memory 24 each have a value which is predetermined from the reference voltage vrefl supplied to the comparator 16 and from a given optical characteristic data of the laser diode 1 which is used as the light source to change the required luminous power to p2 for the laser diode 1 to supply an appropriate quantity of light when the recording dot density is changed to 400 dpi it is necessary to prepare a different reference voltage vref2 other than the reference voltage vref1 and additional reference voltages must be prepared for each of recording dot densities which can be selected with the laser beam printer as it may be assumed that the input output characteristics of the d a converter 21 is given from the experimental data once a digital data input to the d a converter 21 is determined an appropriate value of luminous power p2 may be predicted by making use of the microcomputer 23 and the read only memory 24 according to the present invention only one reference voltage vrefl is prepared and stored in the read only memory 24 for carrying out automatic power control resulting in a stable output of a luminous power to the laser diode 1 when a printing is made with a reference recording dot density used and when a printing is performed with a different recording dot density that is changed from the prepared reference recording dot density the value of the required picture luminous power to the d a converter 21 through the automatic power.