Survivor sequences information is supplied to a RAM 61-1 and a RAM 61-2 as an input Din. The RAM 61-1 and the RAM 61-2 perform an interleaving operation and store the survivor sequences information alternately in accordance with a clock CK1 and a clock CK2 differing in phase from the clock CK1 by half a period. The phases of the clock CK1 and the phase of the clock CK2 are delayed by half a period when a write operation is switched to a read operation with data being outputted at a selector 62 in an appropriate order accordingly. Traced-back data is then inputted from terminal A and terminal B to the selector 62 and outputted from a terminal X after one of these items of data has been selected at a prescribed timing.