A transmitting/receiving system includes two transmitting/receiving apparatuses connected through a transmission cable having two one-direction signal lines. Each apparatus includes a transmitting unit that transmits a signal, a receiving unit that receives a signal, a calculator, a controller, a storage, and an estimating unit. The calculator calculates an Signal to Noise (S/N) ratio of the received signal. The controller controls the calculation by obtaining, as a first value, a strength of a signal caused by a check signal transmitted during absence of a check signal being received, obtaining, as a second value, a strength of a check signal received during absence of a check signal being transmitted, and dividing the second value by the first value. The storage stores relationship between a predetermined transmission-cable S/N ratio and temporal signal fluctuation. The estimating unit estimates temporal signal fluctuation from the calculated S/N ratio and the relationship.