A first transmission apparatus and a second transmission apparatus recognize the working path and the back-up path group from communication paths between the two apparatuses. The first transmission apparatus selects a measurement path from the back-up path group and transmits a test signal via the measurement path to the second transmission apparatus. The second transmission apparatus uses the test signal to measure communication quality, records the measurement data, and transmits a test signal including the measurement data via the measurement path to the first transmission apparatus. The first transmission apparatus records the measurement data included in the test signal from the second transmission apparatus, uses the test signal to measure communication quality, and records that measurement data. The first transmission apparatus selects a new measurement path from the back-up path group and transmits a test signal including that measurement data via the new measurement path to the second transmission apparatus.