Disclosed herein is a decoding apparatus for decoding channel input bits from a partial-response channel output in accordance with a trellis obtained by combining a coding constraint and state transitions of a partial response for a case in which the length of a memory required for describing the coding constraint is greater than the length of a channel memory of the partial response. The apparatus may include a first calculation unit configured to carry out a first calculation on first branch information, which may be defined as information on first branches included in three or more branches merging in a state determined in advance, and first path information defined as information on first paths for the first branches; and a second calculation unit configured to carry out a second calculation on a first calculation value obtained as a result of the first calculation.