At the preceding stage of a first one of two APP decoders that constitute a turbo decoder, there is provided a logarithmic-likelihood computing circuit that creates and outputs logarithmic likelihoods L(y′i|yi) of a reproduced signal y′i which is soft information. The first APP decoder has an APP decoding function of receiving logarithmic likelihoods L(y′i|yi) of the reproduced signal yi as well as a priori information as to code data c′i, updating the code data c′i and producing outputs of a posteriori probabilities according to trellis diagram information that satisfy constraints concerning an RLL modulation. Turbo decoding becomes compatible with the RLL demodulation, and a turbo decoding with high error-correcting capability is usable in reproducing channel data ai recorded onto a recording medium, which allows a recording density of the recording medium to be increased.