A data transmission system comprising a transmitter and a receiver. The transmitter comprises a phase encoder for partitioning consecutive bit data to be input in data in units of X bits; and converting a 2x value indicated by the data of X bits in unique association with an (N/2−1)Y value of a Y symbol, and for confining use of signal points, from among the signal points of N-ary phase, only to a signal point P1 (at a phase angle 0) and N/2−2 signal points P(2n+2) (where 1≦n