1. A coding apparatus for carrying out serially concatenated coded modulation with respect to data input comprising: a first coding means for carrying out coding whose code rate is k/(k&plus;1) with respect to data of k-bit input; a interleaving means for interleaving order of bits constituting data comprising a bit series of (k&plus;1) bits coded by said first coding means; a second coding means serially concatenated with said interleaving means to carry out coding whose code rate is 1 with respect to data of (k&plus;1) bit input; and a mapping means for mapping data of (k&plus;1) bit coded by said second coding means to a transmission symbol of a predetermined modulation, wherein said interleaving means interleaves order of said bits so that at least a part of weights with respect to data comprising a bit series of said (k&plus;1) bit supplied from said first coding means are coded by said second coding means; said second coding means makes as small as possible the total value of the hamming distance of input bits between passes to be the minimum Euclidean distance with respect to data of (k&plus;1) bit supplied from said interleaving means; and said mapping means causes the hamming distance of input bits in said second coding means to correspond to a small one as the distance between signal points on the I/Q plane is smaller to apply mapping to data of (k&plus;1) bit supplied from said second coding means.