PURPOSE: To avoid misrecognition of start and stop conditions and to attain stable serial transmission by making a pulse width of a serial clock(SCL) whose noise is eliminated by a noise eliminating circuit narrow by means of a waveform shaping circuit.

CONSTITUTION: A SCL1d passing through a noise eliminating circuit 4 passes through a waveform shaping circuit 5, a leading is delayed by 2 clocks by a clock (f) to make the pulse width of the SCL1d narrow thereby locating the SCL1d to come at the inside of an SDA1c. Thus, the SDA1c is changed at conditions except the start and stop conditions when the SCL2e is at 'Q', and the misrecognition of the start and stop conditions is avoided to attain stable serial transmission.