PURPOSE: To attain a high speed variable power processing by deciding whether or not a data to be a variable power is a uniform level when the data is transmitted to a variable power circuit, omitting the variable power processing in case of uniform level and returning immediately the data of uniform level to the system by an amount in response to the variable power magnification.

CONSTITUTION: A CPU 1 informs the variable power magnification and a data amount for one line's share in the main scanning direction after a vairable power to a controller 10. When an image data is transmitted from the system to the variable power circuit, the controller 10 controls the address and read/write of an input data line buffer 9 and an output data line buffer 11 depending on the variable power magnification. On the other hand, the data of uniform level is unchanged for the pattern before and after the variable power even if the variable power is processed. Thus, the image data of uniform level is returned immediately to the system while omitting the variable power processing, allowing to shortening the variable power processing time.