Abstract: The present invention relates to a detection of a cutoff condition of an optical fiber. An optical transmission system includes an optical amplifier, an optical/electrical converter, a dispersion compensator or polarization dispersion compensator. An optical amplifying unit is made up of a former-stage optical amplifier for amplifying and outputting an optical signal inputted thereto, a latter-stage optical amplifier for handling the amplified optical signal from the former-stage optical amplifier and a latter-stage input level detecting section for detecting a latter-stage input level representative of an input level of the optical signal inputted thereto; and a control section for controlling an output level from the optical amplifying section based on the latter-stage input level and a reference latter-stage input level. This configuration enables detecting an off-condition of a fiber with a simple circuit and optical parts already existing therein without suppressing an output level of an optical signal.