A transmission circuit operating at a high efficiency and a low distortion is provided. A signal generation section 11 generates a vector signal and an amplitude signal. A vector modulation section 13 performs vector modulation on the vector signal. An amplification section 15 amplifies the signal processed with the vector modulation. A signal processing section 12 performs predetermined signal processing on the amplitude signal and outputs the resultant signal. A regulator 14 controls a voltage to be supplied to the amplification section 15 based on the magnitude of the signal which is output from the signal processing section 12. The signal processing section 12 determines whether or not the amplitude signal exceeds a threshold value at an interval of a predetermined time period, selects a discrete value to be output based on the determination result, and outputs a signal having the selected discrete value.