To reduce the size of the device for judging the error rate on a channel and reducing the amount of processing, a normalization accumulator cumulatively adds the number of times that normalization is performed within a predetermined time in an ACS circuit calculating a state metric. The table stores a table establishing correspondence between the number of times of normalization cumulatively added by the normalization accumulator. The table uses the stored table to judge and output an error rate corresponding to the input cumulative number of times of normalization.
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