Communication system and transmission apparatus

A transmission circuit (120) includes a photo-coupler (130) the conductive/nonconductive states of which are controlled in accordance with data that are to be transmitted to a communication apparatus (200). A reception circuit (140) includes a photo-coupler (150) the conductive/nonconductive states of which are controlled in accordance with the conductive/nonconductive states of a transport path. A transmission circuit (220) includes a photo-coupler (230) the conductive/nonconductive states of which are controlled in accordance with data that are to be transmitted to a communication apparatus (100). A reception circuit (240) includes a photo-coupler (250) the conductive/nonconductive states of which are controlled in accordance with the conductive/nonconductive states of the transport path. The photo-coupler (250) is incorporated on the transport path. The photo-coupler (130) and photo-coupler (250) are in mutually opposite conductive/nonconductive states.