To provide a trapezoidal wave signal generating circuit in which harmonic components included in the waveform can be reduced as much as possible, and an asynchronous serial communication transmitter employing that trapezoidal wave signal generating circuit.

Threshold level voltage generating circuits 37 and 38 generate threshold level voltages equal to integer times of a forward voltage VF with reference to 0V and a power supply voltage VB, respectively. Comparators CP1-CP6 control the output currents from transistors T16-T18 based on the comparison results of (the voltage Vc between the terminals of a capacitor C11+VF) and the threshold level voltages such that charge/discharge currents Ia and Ib decrease as a voltage Vc approaches a lower limit voltage VF or an upper limit voltage (VB-VF). The threshold level voltage is set such that the voltage Vc approaches a sine wave during level variation of transmission data TXD under a power supply voltage VB of 8V.