For the purpose of clarifying the physical nature of the inception of hydraulic cavitation, distilled water saturated with air at an atmospheric pressure was injected as a free jet through small glass nozzles into the same kind of liquid under various states of thermodynamic meta-stability. The condition of the critical states for the inception of cavitation was observed. The result of this experiment seems to show that, contrary to the generally accepted hypothesis of the existence of “nuclear bubbles”, some kind of direct coupling between the state of flow of the liquid and the inception of cavitation therein may be blamed for the phenomenon.