PURPOSE: To improve the data compression rate and to enhance the utilizing efficiency of a recording medium by compressing a data coincident with a data string constituting a prescribed data block into a data string replaced with information of the number of data coincident consecutively, and identification information representing the location of the information of a data number in the replaced data string.

CONSTITUTION: When a data of a 1st block is inputted entirely to a character compression circuit 6 repetitively, a control circuit 10 switches a signal line 17 and an output of a selector 5 is used as a data from a selector 18. When a data string of a 2nd block is inputted, a processing 9 compares it with an input data string of the 1st block stored in a buffer memory 3. When they are coincident, a signal line 12 is valid and an input data string of the 1st block and the same data of the input data string of the 2nd block inputted to a same data counter 13 generate the information of consecutive length. The data count inputted to the compression circuit 6 is compressed, and written in a buffer memory 8 through a selector 7.