PURPOSE: To stabilize the initial start operation of decoding regardless of the state of a communication line by preparing plural Fano metric tables and sequentially selecting one of the tables in the case of the initial start of decoding when the frequency of generating a skip signal showing a decoding state exceeds a threshold value.

CONSTITUTION: The Fano metric tables to various line states are prepared in the memory 11 and in the case of the initial start of a decoding operation, a control signal processing circuit 12 judges whether the frequency of generating the skip signal exceeds the prescribed threshold value as the reference value of a normal decoding operation state or not. During an abnormal state exceeding this threshold value, a select signal is applied to a table selecting circuit 13 and the Fano metric tables are sequentially switched. Therefore, a normal decoding operation is enabled so that the frequency of generating the skip signal is lower than the threshold value. Thus, the initial start operation of decoding can be stabilized regardless of the state of the communication line.