1. A communication system that sends and receives signals via two adjacent channels in which interchannel interference exists, comprising: a transmitter device that transmits data of one unit which consists of a plurality of bits constituting an in-phase component that has been obtained by multi-value QAM modulation, from one channel, and transmits data of one unit which consists of a plurality of bits constituting a quadrature component that has been obtained by the multi-value QAM modulation, from other channel; a receiver device provided in each channel that generates a soft decision target value for each of a plurality of received bits inputted from the corresponding channel and makes a soft decision with respect to the plurality of received bits by using the respective soft decision target values; and means for inputting the soft decision target values of the plurality of received bits of the receiver device of one channel to the receiver device of the other channel, wherein the receiver device adjusts the respective soft decision target values of its own channel by using the respective soft decision target values of the plurality of bits inputted from the receiver device of the other channel and makes a decision with respect to the received bits on the basis of the soft decision target values.