PURPOSE:To easily detect the frequency and level of a short-time continuing wave by starting a sweep for synchronism when an input wave exceeding a set level arrives and is still inputted even a constant time later. CONSTITUTION:A noise detecting means 13 connected to the communication line 7 and power line 9 of a device to be monitored and an antenna 14 which is installed nearby a device 6 are connected to the environmental noise recording device 12 and the fault occurrence level of the device 6 to noises from respective paths is set previously. Then the entering noises begin to be monitored, and the sweep is started if a noise which has a specific frequency at the time of the arrival of a noise larger than the level and stops the certain time later still exceeds the set level even the constant time later at either or both of capacitance and inductance forming a resonance circuit. Then the frequency and level, and crest value and level are printed out together with the arrival time to know the kind, level, frequency, time, etc., of a noise which causes fault occurrence.
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