A device for estimating the error rate of a transmission line has a small scale. To reduce the computational complexity. A normalization counting circuit counts the normalizations performed in a predetermined time by an ACS circuit for calculating a state metric. A table contains the number of normalizations counted by the normalization counting circuit and the error rate of a transmission line correlated with the number of normalization. The table finds and outputs the error rate corresponding to the inputted number of normalizations by using the table.
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