To relieve the load attended with performance monitor processing by a section terminator of a synchronous digital hierarchy(SDH) transmission network, a bus terminator, a management network and a network monitor.

Assignment state flags 11, 12 set/released by a network monitor equipment 24 are provided to performance monitor sections 26, 28 provided to termination sections 25, 27. When the assignment state flags denote assignment release, the termination section stops an output of a performance monitor notice. A current data processing section 29 stops count of performance data, comparison with a threshold level, and transfer of count to a history data storage section 30 when no performance monitor notice is received within a prescribed time. The history data storage section 30 stops generation of history data when no count is transferred even after lapse of an observation period.