A variable power supply voltage generator generates a variable power supply voltage Vvar and supplies it to other circuits. A transmitting circuit 130 (or 140), operative at the variable power supply voltage Vvar, generates multi-value analog signals Smulti and transmits them to other circuits. A receiving circuit 140 (or 130), operative at the variable power supply voltage Vvar, receives the multi-value analog signals Smulti and subjects them to A/D conversion to generate multi-value digital signals. The threshold voltage generator generates threshold voltages used for A/D conversion from the variable power supply voltage Vvar or from a signal having a voltage value proportional to that of the variable power supply voltage Vvar and supplies them to the receiving circuit. An analog clock generator 120 generates an analog clock signal having a cyclical analog waveform. In response to the analog clock signal, a clock pulse generator 150 generates a clock pulse signal having a cycle period of 1/N of the cycle period of the analog clock signal. The clock-synchronous circuits 130, 140 operate synchronously with the clock pulse signals generated by the clock pulse generator 150.