We propose a parallel combinatory multiple-subcarrier (PC-MS) optical communication system. The proposed system with N subcarriers selects at each symbol interval a subset of U subcarriers and transmits ⌊log2 (N/U)⌋ bits for the selected subset. The other N - U = Npc subcarriers are modulated by points from an M-PSK signal constellation. Thus, the proposed system can transmit more information at each symbol interval than the conventional MS system with N subcarriers. In addition the proposed system can decrease the bias that must be added to the MS electrical signal, because the number of subcarriers other than null subcarriers is smaller than that in the conventional MS system and thus the minimum power is larger. Therefore, the proposed system can decrease the required energy per bit.