A transmitting device includes: a first transmission unit; a second transmission unit having transmission lines each having a proximal end at which a transmitting-side connecting portion is provided; and a first switch for selecting a transmission line from the second transmission unit, to connect the selected transmission line to the first transmission unit. A receiving device includes: a third transmission unit having transmission lines each having a distal end at which a receiving-side connecting portion is provided; a fourth transmission unit; a second switch for selecting, from the third transmission unit, a transmission line including the receiving-side connecting portion optically connected to the transmitting-side connecting portion, to connect the selected transmission line to the fourth transmission unit; and a detection unit configured to detect whether a connection between the transmitting-side and receiving-side connecting portions is abnormal based on light amount of an optical signal transmitted through the fourth transmission unit.