PURPOSE: To facilitate store judgement regarding plural line buffers by using a counter which counts up or down by one every time data is written in or read out of each line buffer.

CONSTITUTION: Every line of data (unit data) is written in a buffer memory 1 having line buffer parts 1WN according to the contents of a counter 2. On the writing of each line of data, a line buffer controlling part 3 allows the counter 2 to count up by one. Further, the reading of every line of data from the buffer memory 1 is carried out according to the contents of the counter 2. Every time, one line of data is read out, a line buffer data processing part 4 decreases the contents of the counter 2 by one.