An input bit error ratio estimating method, comprising: a computing step of computing an internal decoding bit inversion ratio and a post-internal decoding residual error detection ratio based on a signal for monitoring a decoding result of decoding processing of an error correction code; a condition determining step of determining which of a plurality of conditions set in advance to be narrowed down to one has been established based on the post-internal decoding residual error detection ratio; an input BER estimating step (ST3, ST4, ST5) of selecting one of a plurality of processing procedures for estimating an input BER, based on one of the plurality of conditions that is determined in the condition determining step as one that has been established, and executing the selected one of the plurality of processing procedures; and an estimation result outputting step of outputting a result of the one of the plurality of processing procedures executed in the input BER estimating step, wherein the internal decoding bit inversion ratio and the post-internal decoding residual error detection ratio are computed in the computing step by the following operations: D r = D / F; and E r = E / F ⋅ N, Where D r represents the post-internal decoding residual error detection ratio, D represents a post-internal decoding residual error count, F represents a frame count, E r represents the internal decoding bit inversion ratio, E represents a decoding inverted bit count, and N represents a bits-per-frame count, and wherein the plurality of conditions in the condition determining step comprise the following Conditions A to C: Condition A: D r <T r; Condition B: T r ≤D r <1.0; and Condition C: D r =1.0, where T r represents a condition determining threshold, and wherein the input BER estimating step (ST3, ST4, ST5) comprises: estimating the input BER as the internal decoding bit inversion ratio when Condition A is established; estimating the input BER by correcting an error of the internal decoding bit inversion ratio to the input BER when Condition B is established; and processing the input BER as unestimable when Condition C is established. a computing step of computing an internal decoding bit inversion ratio and a post-internal decoding residual error detection ratio based on a signal for monitoring a decoding result of decoding processing of an error correction code; a condition determining step of determining which of a plurality of conditions set in advance to be narrowed down to one has been established based on the post-internal decoding residual error detection ratio; an input BER estimating step (ST3, ST4, ST5) of selecting one of a plurality of processing procedures for estimating an input BER, based on one of the plurality of conditions that is determined in the condition determining step as one that has been established, and executing the selected one of the plurality of processing procedures; and an estimation result outputting step of outputting a result of the one of the plurality of processing procedures executed in the input BER estimating step, wherein the internal decoding bit inversion ratio and the post-internal decoding residual error detection ratio are computed in the computing step by the following operations: D r = D / F; and E r = E / F ⋅ N, Where D r represents the post-internal decoding residual error detection ratio, D represents a post-internal decoding residual error count, F represents a frame count, E r represents the internal decoding bit inversion ratio, E represents a decoding inverted bit count, and N represents a bits-per-frame count, and wherein the plurality of conditions in the condition determining step comprise the following Conditions A to C: Condition A: D r <T r; Condition B: T r ≤D r <1.0; and Condition C: D r =1.0, where T r represents a condition determining threshold, and Condition A: D r <T r; Condition B: T r ≤D r <1.0; and Condition C: D r =1.0, where T r represents a condition determining threshold, and wherein the input BER estimating step (ST3, ST4, ST5) comprises: estimating the input BER as the internal decoding bit inversion ratio when Condition A is established; estimating the input BER by correcting an error of the internal decoding bit inversion ratio to the input BER when Condition B is established; and processing the input BER as unestimable when Condition C is established. estimating the input BER as the internal decoding bit inversion ratio when Condition A is established; estimating the input BER by correcting an error of the internal decoding bit inversion ratio to the input BER when Condition B is established; and processing the input BER as unestimable when Condition C is established.