At the very beginning stage of cavitation an infinitesimal cavity suddenly emerges and rapidly grows on the wall close to the point of flow separation. The present paper intends to make clear the mechanism of this peculiar phenomenon previously discovered by the present authors in oil flows, by experimentally examining the characteristics of a separating flow. First, pressure and temperature distributions along a cylindrical smooth wall in an oil flow were measured across the point of separation, which has proved that both tensile stress and heat are really generated near the separation point. Second, a laminar air flow passing over a sharp edge was inspected by particle image velocimetry (PIV). The inspection of the separating flow based on the PIV data has revealed that fluid particle near the separation point is not only drastically stretched but also intensely sheared along the separation streamline. With all the results integrated, it has been concluded that the singular structure of a separating flow is the real cause of both tensile stress and heat generation. When thus generated tension exceeds a threshold, it possibly breaks the adhesion between the liquid and the wall, leaving a microscopic rift on the interface that would grow to a visible cavity.