One or more embodiments provide a decoding technique (and its approximate decoding technique) enabling a stable operation even if a noise variance is low at the implementation with a fixed-point arithmetic operation having a finite dynamic range. A technique is provided for causing a computer to perform calculation using a sum-product decoding method (belief propagation method) with respect to LDPC or turbo codes. For calculating an update equation of a log extrinsic value ratio from an input, a (separated) correction term is prepared obtained by variable transformation (scale transformation) of the update equation so that the update equation is represented by a sum (combination) of a plurality of terms by transformation of the equation and a communication channel noise variance is a term separated from other terms constituting a sum of a plurality of terms as a term to be a factor (scale factor) by which a log is multiplied. With an estimated communication channel noise variance as an input, the (separated) correction term is approximated by a simple function so as to cause the computer to make calculation (iteration) on the basis of a fixed point on bit strings of finite length (m,f: m is the total number of bits and f is the number of bits allocated to the fractional part).