An optical signal receiver is provided which is capable of making an optical signal receiver come into common and shared use even when a plurality of transmission methods is to be performed and of constructing a simplified optical transmission system. In a performance monitoring circuit, a switching control section transmits a connecting control signal to make a switching section operate so that the switching section selects a monitoring section to be connected based on bit-rate information corresponding to information about a transmission method type fed from a frequency detecting circuit and the selected monitoring section is connected to a clock/data regenerating circuit. The monitoring section extracts alarm information from a data signal fed from the clock/data regenerating circuit and transfers the alarm information to the selection connecting section. When the switching control section receives alarm information, the alarm information is transferred to devices mounted on a downstream side.