PURPOSE: To improve the compression ratio of character data by catching a common part in a direction from a front character to a rear character and symbolizing a common character string while observing the front characters from the final character.

CONSTITUTION: One word in a word group arranged in a prescribed order is compared with a word preceding to this word, and a character string part having more than two coincident characters is turned to an omitted code corresponding to the number of coincident characters. The remaining character strings are rearranged reversely, the rearranged character string is compared from the front character, the common character string is stored, and the word end part is expressed by this stored position code. Thus, by mixing a method to compress character data while observing the character string from the front and a method to compress character data while observing the character string from the back, compression ratio can be improved.