An encoder with error correction for usc in a transmitting apparatus which sends data in a 2 n QAM system, wherein n is an even number equal to or more than four, comprising: a converting section (1C - 1F) converting said data into (n-1)-bit parallel data; a convolutional encoder receiving only one bit of the (n-1)-bit parallel data; and a signal point generator (3B - 3E) receiving 2-bit data outputted from said convolutional encoder and the (n-1)-bit parallel data excluding the one bit of the (n-1)-bit parallel data, characterized in that: said signal point generator generates multilevel signals according to the 2 n QAM system, wherein the 2-bit data from said convolutional encoder and the (n-1)-bit parallel data excluding the one bit are both separated into an in-phase component and a quadrature component, each of which only affects corresponding component of coordinate of the signal point in a signal point plane of an in-phase amplitude component - quadrature amplitude component coordinate system, an identical bit layout is assigned to in-phase components at signal points having an identical in-phase amplitude component and an identical bit layout is assigned to quadrature components at signal points having an identical quadrature amplitude component; mutually different bit values are alternately assigned to the bit corresponding to an in-phase component output from said convolutional encoder for signal points adjacent to each other in a direction of an in-phase amplitude component axis; mutually different bit values are alternately assigned to the bit corresponding to a quadrature component output from said convolutional encoder for signal points adjacent to each other in a direction of a quadrature amplitude component axis. a converting section (1C - 1F) converting said data into (n-1)-bit parallel data; a convolutional encoder receiving only one bit of the (n-1)-bit parallel data; and a signal point generator (3B - 3E) receiving 2-bit data outputted from said convolutional encoder and the (n-1)-bit parallel data excluding the one bit of the (n-1)-bit parallel data, characterized in that: said signal point generator generates multilevel signals according to the 2 n QAM system, wherein the 2-bit data from said convolutional encoder and the (n-1)-bit parallel data excluding the one bit are both separated into an in-phase component and a quadrature component, each of which only affects corresponding component of coordinate of the signal point in a signal point plane of an in-phase amplitude component - quadrature amplitude component coordinate system, an identical bit layout is assigned to in-phase components at signal points having an identical in-phase amplitude component and an identical bit layout is assigned to quadrature components at signal points having an identical quadrature amplitude component; mutually different bit values are alternately assigned to the bit corresponding to an in-phase component output from said convolutional encoder for signal points adjacent to each other in a direction of an in-phase amplitude component axis; mutually different bit values are alternately assigned to the bit corresponding to a quadrature component output from said convolutional encoder for signal points adjacent to each other in a direction of a quadrature amplitude component axis. said signal point generator generates multilevel signals according to the 2 n QAM system, wherein the 2-bit data from said convolutional encoder and the (n-1)-bit parallel data excluding the one bit are both separated into an in-phase component and a quadrature component, each of which only affects corresponding component of coordinate of the signal point in a signal point plane of an in-phase amplitude component - quadrature amplitude component coordinate system, an identical bit layout is assigned to in-phase components at signal points having an identical in-phase amplitude component and an identical bit layout is assigned to quadrature components at signal points having an identical quadrature amplitude component; mutually different bit values are alternately assigned to the bit corresponding to an in-phase component output from said convolutional encoder for signal points adjacent to each other in a direction of an in-phase amplitude component axis; mutually different bit values are alternately assigned to the bit corresponding to a quadrature component output from said convolutional encoder for signal points adjacent to each other in a direction of a quadrature amplitude component axis.