A decoding apparatus, decoding the LDPC code using a message passing algorithm, sets a message as a log likelihood ratio having, as a base, a real number “a” which is a power of 2, and includes a check node processing calculating unit for receiving a message from a bit node to calculate a message from a check node. The check node processing calculating unit includes a converter for converting an absolute value x of the message to output f (x) and a converting unit supplied as an input y with a sum of the absolute values x of the message from the totality of the bit nodes less one, converted by the converter, subdividing the input y in preset domains and for converting the number in the domain into g (y), and expresses the boundary values of the domains of the input y and f (x) by a power of 2.