A transmission circuit that performs modulation based on a phase difference signal and an amplitude signal includes an asymmetrical phase rotation device. The asymmetrical phase rotation device performs an operation of subtracting 2π from a value of the phase difference signal when the value of the phase difference signal is greater than a predetermined positive threshold value, or an operation of adding 2π to the value of the phase difference signal when the value of the phase difference signal is less than a predetermined negative threshold value. Accordingly, the transmission circuit has distortion reduction characteristics improved uniformly over a range of frequencies higher or lower than a carrier wave band.