When a failure detection server calculates a measurement average value at every predetermined time with respect to measured values collected in the past, and collects measured values from cable modems, the server sets a failure determination threshold value by adding or subtracting a predetermined difference threshold value to or from the measurement average value so as to determine the presence of failure. A failure analysis server performs a screening process of inputting determined failure information and eliminating logically conflicting failure information, obtains a differential of the screened failure information between this time and previous time so as to extract changed failure information and newly generated failure information, and then, repeats state determination at an upstream connection point, starting from a cable modem having failure information, so as to estimate the failure generated location.