A waveform observation system includes two communication nodes, a waveform observation apparatus, and a signal generation portion. The two communication nodes execute a full-duplex communication by a differential signal through a transmission line. The waveform observation apparatus observes a communication signal waveform in the transmission line in response to an input of a trigger signal. The signal generation portion outputs the trigger signal. One of the two communication nodes generates a clock signal, and transmits a signal in synchronization with the clock signal. Remaining one of the two communication nodes reproduces the clock signal included in the signal received from the one of the two communication nodes, and transmits a signal in synchronization with the clock signal that is reproduced. The signal generation portion outputs the trigger signal when equal to or more than two symbols indicated by the signal output to the transmission line consecutively coincide with one another.