To solve the problem that it is necessary to synchronize operating clocks in serial communication through a semiconductor integrated circuit between a receiving side and a transmitting side.

In a circuit of the transmitting side, when data from a serial I/O is '0', a second reload register is set by a reload register control part into a timer after setting of a first reload register, and when the data is '1', the first register is set by the control part into the timer after setting of the second register, so that waveforms with different duty ratios are outputted, in a circuit of the receiving side, the waveforms are converted into data '0' and '1' at a ratio of waveforms between a 'H' level interval and a 'L' level interval by a data conversion part to input into the serial I/O.