To perform efficient high compression encoding.

A CPU turns a leading character to a character string 1 in a step S1, judges whether or not a character before encoding remains in the step S2, encodes the character string 1 and ends the processing in the step S3 in the case that it does not remain and adds one character α following the character string 1 to the character string 1 and turns it to the character string 2 in the step S4 in the case that it remains. The CPU judges whether or not the character string 2 is registered in a dictionary in the step S5, turns the character string 2 to the character string 1 in the step S6 and returns to the step S2 in the case that it is registered and encodes the character string 1 in the step S7 in the case that it is not registered. The CPU moves the character string 1 to the head of the dictionary in the step S8, registers the character string 2 to the head of the dictionary in the step S9, turns the character αto the character string 1 in the step S10 and returns to the step S2.