In this paper, we will propose a digital watermarking for voice signals recorded by a digital voice recorder especially in conferences. We will discuss some requirements for detecting falsifications in the voice signal, because the conference record has the probability that a conclusion is changed for only a falsification in a little time interval. Next, we will show a method of locating an altered place in the voice signal and a new idea applying a white Gaussian noise for guarantee of the valid voice signal without falsification. One characteristic of our proposal is an agitation depending on a peak spectrum for making it difficult to illegally change the embedded watermark.