In order to improve usage efficiency of a RAM provided in a video controller of a laser printer and to prevent occurrence of print overrun errors, a packet data, which contains character/symbol code data and print position information thereof, is developed into a printable format data on a band basis, which is then compressed and stored in an image data buffer provided in the RAM. The compressed image data is retrieved and decoded to restore the original printable format data. The latter data is stored in a print buffer and then sent to the print mechanism for printing. When the image data buffer does not have sufficient memory space for storing subsequent one band's worth of the packet data in compressed form, the data which has already been processed is erased to secure free area of the image data buffer. Alternatively, resolution of the printable format data stored in the image data buffer in compressed form is reduced to secure free memory space in the image data buffer.