A transmission device includes a first mapper, a second mapper, a converter, a superposer, and a transmitter. The first mapper is configured to map a first bit stream of a first data series to generate a first modulated symbol stream. The second mapper is configured to map a second bit stream of a second data series to generate a second modulated symbol stream. The first modulated symbol stream and the second modulated symbol stream are representable on a complex plane extending in a first direction and a second direction. The converter is configured to convert the second modulated symbol stream in accordance with the first modulated symbol stream only in the first direction on the complex plane. The superposer is configured to superpose the first modulated symbol stream and the second modulated symbol stream converted by the converter, at an amplitude ratio, to generate a multiplexed signal.