1. An apparatus comprising: a hard decision value calculator to calculate hard decision values from coordinates of a received symbol having been subjected to multilevel modulation; an approximate value calculator to calculate, from the coordinates of the received symbol, approximate values of log-likelihood ratios to be used for soft decision decoding; and an approximate value corrector to determine whether there is a conflict between a sign bit of an approximate value calculated by the approximate value calculator and a hard decision value calculated by the hard decision value calculator, invert the sign bit of the approximate value in a case where it is determined that there is the conflict between the sign bit of the approximate value and the hard decision value, and output the sign-bit-inverted approximate value as a soft decision value.