PURPOSE: To perform the compression expanding processing of the picture data string by a simple circuit configuration with less distortion by turning the ratio of the number of data between the input data string and the output data string to a simple integral ratio in performing the compression expanding processing of the picture data string.

CONSTITUTION: For example, the sampling point (FS1=4fsc) with 100 picture elements of a side panel part is thinned out to 25 picture elements (FS2=4 fsc×25/100) of an over scanning part. In this case, the compression ratio is 100:25, that is, 4, and the ratio of the I/O data is the simple integral ratio. Accordingly, in this case, only one data should be thinned out and taken out for four input data by the compression circuit 1. Therefore, to make all the output data strings is quite simple and it is unnecessary to calculate the insertion data by using the expression of the insertion data. The compressed FS2 data string is obtained by the interpolation of four data per one data in reverse.