predetermined gate time t and to indicate in the indicator 132 that the gate time t is excessively long now when the raw noise waveform is as shown in fig1 a this raw waveform is attenuated or amplified by the attenuator 10 or the amplifier 12 which is suitably selected by the change over switch 14 envelope detected by the envelope detecting circuit 16 and comes to be an envelope detected waveform as shown in fig1 c when this envelope waveform reaches a preset level or more the controller 130 starts the a d converter 18 and the frequency counter 124 makes access to the ram 39 s 20 and 126 and an a d converter waveform as shown in fig1 d is stored in the ram 20 the controller 130 repeatedly performs the above described operation a certain number of times predetermined by the storage capacity of the ram 20 by this in the ram 20 there are stored the envelope noise waveforms after the envelope reaches a certain level or more until a preset period of time elapses on the other hand the frequency counter 124 counts frequency of the noise raw waveform as shown in fig1 a for a predetermined gate time t shown in fig1 b for example and the result is stored in the ram 126 additionally when the envelope waveform does not continue for the gate time t the value measured by the frequency counter 124 is cancelled by the controller 130 and the information of that the gate time t is excessively long is indicated in the indicator 132 on the other hand in performing the reproduction of the noise waveform the controller 130 makes access to the ram 39 s 20 and 126 in response to.