PURPOSE: To enable fast processing without generating any mask data even when data written in a rotation register does not reach capacity by clearing the rotation register to 0 after reading data of a 90°-rotated pattern out.

CONSTITUTION: When the data are written in the rotation register(RR) 3, a trigger generating circuit 8 turns on a flip-fop(FF) 9 and places a counter 10a in operation. A read pulse is sent to the RR3 from a gate 11a according to the conditions between the output of the FF 9 and counter 10a to output the data. A gate 11d sends a set pulse to a register 4 to latch the output data of the RR 3. Then the gate 11b sends a write signal to the RR 3 and a gate 11c outputs 0 data from the 0-clear register 6 at nearly the same time; and a selector 2 selects this output and writes it in the RR 3. Then the FF 9 is turned off with the output of the counter 10a.