A liquid Li free-surface stream flowing at 15 m/s under a high vacuum of 10−3 Pa is to serve as a beam target (Li target) for an intense neutron source such as the planned International Fusion Materials Irradiation Facility (IFMIF). This study examines cavitation-like acoustic noise occurring upstream of the Li target. This noise is detected using an acoustic emission sensor that is installed at a Li target assembly in which the Li target was produced. A time-frequency analysis by continuous wavelet transform (CWT) was performed to characterize the acoustic noise, which determined the cause of the acoustic signal was cavitation. And the occurrence of cavitation was discussed by using Bernoulli's equation and compared with the experimental observations. As a consequence, we revealed a criterion of inception of cavitation and a proper startup pressure of the Li target, which will be a design basis for the future IFMIF Li target facility.