An optical communication system includes a looped path in which an active trunk fiber and a standby trunk fiber are connected in a loop. A plurality of optical combining/splitting units is provided on an active trunk fiber in series, and a plurality of slave devices is connected the plurality of optical combining/splitting units, respectively. A master device performs optical communication with each slave device. Each optical combining/splitting unit includes an optical sensor configured to detect an optical signal passing through an optical fiber and a transmitter. The transmitter determines whether or not there is an abnormality in optical communication on the basis of a detection state of the optical signal and transmits an abnormality notification including identification information of the optical combining/splitting unit to a monitoring apparatus in a case where there is an abnormality. The monitoring apparatus holds connection relationship information indicating a connection relationship between the plurality of optical combining/splitting units along the active trunk fiber. In a case where the monitoring apparatus receives the abnormality notification from at least one of the plurality of optical combining/splitting units, the monitoring apparatus specifies a failure position on a communication path on the basis of identification information of each optical combining/splitting unit that has transmitted the abnormality notification and the connection relationship information.