PURPOSE: To provide a data transmission device which allocates the code of short bit length to a transmission symbol with high generation frequency, executes encoding/decoding with variable length and reduces transmission information quantity.

CONSTITUTION: When a transmission symbol with high generation frequency, which is detected by a variable length encoder 1, is not held in an adaptive code book 3, it is encoded, transmitted to a transmission line 2 and is held in the adaptive code book 3. When the detected transmission symbol is held in the adaptive code book 3, the code of short bit length is allocated to the symbol and is transmitted to the transmission line 2. The reception symbol is decoded from the code transmitted by a variable decoder 4. When the reception symbol is not held in an adaptive code book 5, the reception symbol is obtained, and it is held in the adaptive code book 5. When it is held in the adaptive code book 5, the reception symbol corresponding to it is obtained.