PURPOSE: To improve the data processing speed of the picture data processing unit having a decoding function and a reduction function of picture data.

CONSTITUTION: A decoding circuit 12 receives compressed picture data from a memory 11 under the control of a control circuit 15 to decode the picture data before compression. The decoded picture data are written in a line buffer 13 and given to a reduction circuit 14 simultaneously. Upon the receipt of the picture data from the decoding circuit 12, the reduction circuit 14 executes logic arithmetic operation to reduce the picture at a prescribed reduction rate. A memory control circuit 15 checks a bit number of the picture data outputted from the reduction circuit 14 and writes the result to the memory 11 every time the bit number reaches a prescribed bit number. In the picture data processor, after the decoding processing, the picture data are directly given to the reduction circuit 14 without being stored in the memory 11 and the reduction processing is executed, then the processing time is reduced.