A check node processing unit updates an extrinsic value ratio based on a prior value ratio for each row of a parity check matrix with respect to input data. An identifying unit identifies, based on an element of the parity check matrix that can be identified by a row and column associated with the updated extrinsic value ratio, a next-target element in the same column and in a different row. The identifying unit identifies an element to be updated in the next step by the check node processing unit, from among multiple elements included in the same column. A variable node processing unit updates, based on the extrinsic value ratio, a prior value ratio associated with the identified next-target element after the check node processing unit completes the updating of each row. The check node processing unit and the variable node processing unit alternately and iteratively execute their operations.