PURPOSE: To output expanded image data while efficiently excluding blank lines from them by detecting the blank lines from the bit image data and excluding the blank lines based on previously set rules.

CONSTITUTION: When a printing instruction is inputted from a keyboard 3, a CPU 1 reads bit image data in a main RAM 10 and transfers them to a blank line detection part 11. The blank line detection part 11 checks whether over one bit of on dots to be printed for each dot sequence of one bit width parallel to the scanning direction of a serial printer 5 are existent in these bit image data or not and when over one bit of on dots are existent in the line as a result, data on this line are dispatched to a printer control part 8 as they are and printed from the serial printer 5. When only one bit of on dots are not existent on the line, however, the line is regarded as the blank line and handled as it is absent.