When a convolution code is decoded, electric power consumption is suppressed keeping error correction capability. In a Viterbi decoder which decodes received signal, a convolution code, having plural series with a soft decision Viterbi decoding method, an estimation control unit estimates quality of the received signal and outputs a control signal according to the quality to a branch metric calculation data obtaining unit. The branch metric calculation data obtaining unit performs logical combination operation between digital multi-value data expressing amplitude of the received signal and the control signal, and thereby, outputs the digital multi-value data directly to a decoding execution unit if the quality of the received signal is lower than a prescribed level, and outputs the digital multi-value data reduced by series each as branch metric calculation data to the decoding execution unit if the quality of the received signal is no less than the prescribed level.