A subscriber premises side optical network unit, and an optical transmission system having the same, is provided that monitors whether or not a communication condition is established with a center side optical line termination and halts signals to be outputted to external nodes when the communication condition is not established. In an optical network unit (ONU), a signal output control means monitors whether or not the communication condition with an OLT is established and when it determines that the communication condition is not established, outputs an output halting request to loopback setting sections to control to assume a loopback state. Thereby, it becomes possible to halt the output of the signals from an interface module to a switch.