An endoscope system includes: an image sensor, a first photoelectric sensor configured to convert an imaging signal and test data output from the image sensor into an optical signal and output the optical signal, optical fibers configured to transmit the optical signal output from the first photoelectric sensor, and an information processing device. The information processing device includes a second photoelectric sensor configured to receive the optical signal transmitted through the optical fibers, and is configured to: measure a current value corresponding to a receiving sensitivity of the test data converted into an electric signal by the second photoelectric sensor； measure a bit error rate of the test data； determine a transmission level of the test data based on the current value and the bit error rate； and assume reasons for a decrease in the transmission level and cause the transmission level to recover based on the transmission level.
