PURPOSE: To provide a decoding device capable of easily deducing run length and mode from a encoded bit train.

CONSTITUTION: Table groups 4 to 4 which are obtained by dividing a table showing the correspondence between code words and the run length and mode indicated by the code words into several groups corresponding to what position a first bit '1' are placed at in the encoded bit train, are provided. A discrimination part 2 detects what position the first bit '1' is placed at in the encoded bit trains and selects the corresponding table 4 out of the table groups 4 to 4. Then a decoding part 5 deduces the run length and mode indicated by the encoded bit train from the selected table 4.