A method and apparatus for mapping a stream of bits output from an encoder to a stream of bits for 2m-ary modulation. The method and apparatus divides the stream of bits from the encoder into a first period and a second period. The first period includes bits with higher priority in the stream of the bits output from the encoder, and the second period includes bits with lower priority. The method and apparatus maps the bits existing in the first period to bit positions with higher reliability in a stream of m bits representing each of the bits for 2m-ary modulation, and maps the bits existing in the second period to bit positions with lower reliability in the stream of m bits.