The pulse, the sing-around and the standing wave methods are mainly applied to under water sound velocity measurements. These methods are related to the correlation method, in that the error in measurement increases as the frequency is lower when the measuring distance is fixed. Hence, we examined the system shown in Fig. 1 as a method which eliminates these weak points. This measuring system can be explained as follows:A PM signal(a phase modulated wave)is made whose modulating signal consists of the addition of the low frequency emitting signal s(t)(Gaussian noise)and receiving signal y(t), and then its auto-correlation function is computed. From this function we can detect the acoustical delay time of the phase-modulating signal s(t)+y(t). Simultaneously, the error in measurement is decided, almost independently of the frequency of s(t), mainly by the carrier frequency ω_c and modulation index m_p of PM signal whose values can be chosen voluntarily after the reception. Accordingly, when ω_c and m_p are larger, the error in measurement is smaller and when ω_c and m_p are fixed, the error is also fixed even though we change the measuring frequency bands of s(t). In other words, it means that instead of s(t), the PM signal can be considered as though it were the measuring signal. This method can be also considered to be qualitatively equivalent to the system which computes the cross-correlation function of s(t) and y(t) and amplifies it non-linearly in order to increase the resolving power, as shown in Fig. 2, 3. Besides, if the reflected sounds are a few and if the external noise contains no periodic components and its equivalent bandwidth is more than the degree of s(t), the characteristics of the correlation method are maintained in this method. Namely, we can take measurements without the influence of external noise and also distinguish the direct sound from reflected sounds.