An optical signal propagating through a transmission line of a multimode fiber is branched by an MMF coupler of a reception device, and the intensity of the signal is detected by a MMF monitor PD. After connecting from the multimode fiber to a single-mode fiber, the optical signal is branched by a SMF coupler, and the intensity of the optical signal after the connection by a SMF monitor PD is detected. A coupling loss is calculated from the two detection results and the status of the occurrence of an error rate is estimated from the obtained coupling loss.