A turbo decoding method is capable of realizing high-speed and highly accurate decoding operations by improving initialization of path metric values for parallel decoding processing. Turbo encoded code data is divided into a first to an N-th sub-code blocks and parallel decoding processing on these sub-code blocks is performed. “A priori initialization processing unit” s are provided which employ a final calculated value in a preceding sub-code block as an initial value for calculation of a path metric value of each sub-code block excluding the first sub-code block in a forward direction and a final calculated value in a following sub-code block as an initial value for calculation of a path metric value of each sub-code block excluding the N-th sub-code block in a backward direction. After the a priori initialization processing of path metric values, parallel decoding processing on each sub-code block is performed.