to bypass the second optical switch 352 by including the first to sixth optical circulators 361 362 363 364 365 and 366 as above the forward directional optical signal and the reverse directional optical signal although sharing a bidirectional link independently circulate the optical recirculation loop so that the optical recirculation loop is available for a bidirectional transmission system in the operation of the control unit 390 the control unit 390 controls the third optical switch 353 and the first optical switch 351 to operate in the difference states that is from a time point at which t 0 to a time point at which t 1t the third optical switch 353 is in the u201c on u201d state while the first optical switch 351 is in the u201c off u201d state from the time point at which t 1t to the time point at which t nt the third optical switch 353 is in the u201c off u201d state while the first optical switch 351 is in the u201c on u201d state it can be seen that a period when the third optical switch 353 remains in the u201c on u201d state is equivalent to a time t needed for the optical signal to proceed in the optical link 380 in the optical recirculation loop the control of the first second third and fourth optical switches 351 352 353 and 354 by the control unit 390 that are not described above will be described with reference to fig2 the control unit 390 generates a control signal to control the odd number channel measuring instrument 391.