1. A method for causing a computer to perform steps of calculation using a belief propagation method as a part of a process for decoding at least one of LDPC and turbo codes, the method comprising the steps of: preparing a separated correction term obtained by variable transformation of an update equation of a log extrinsic value ratio in the belief propagation method, for calculating the update equation, so that: the update equation is represented by a sum 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 by which a log is multiplied; and approximating the separated correction term by a simple function so as to cause the computer to make iterative calculation on the basis of a fixed point on bit strings of finite length with an estimated variance of a communication channel noise as an input.