A phase of a symbol on an inner or outer circle of a received 8QAM symbol set, which is bitmapped by being assigned one bit depending on whether the symbol is on the inner or outer circle of a constellation mapping diagram and two bits for each of the quadrants, is rotated to generate, by QPSK likelihood generation, along with a likelihood of a received QPSK modulation symbol, a likelihood for the two bits assigned to the quadrant of the 8QAM modulated symbol set whose phase is rotated. Further, the phase-rotated 8QAM modulated symbol set is phase-rotated to the first quadrant of the diagram to move the symbol set to the QPSK symbol mapping positions, thereby generating, by QPSK likelihood generation, the likelihood for the one bit assigned to represent whether the symbol is on the outer circle or the inner circle of the 8QAM modulated symbol set.