Incipient cavitation in a contoured-plug valve flow is studied both in a high-inlet-pressure range of 3∼5MPa and in a low-pressure range of around 0.4MPa. The argument is focused on features of a string bubble cavitation on schlieren photography. In a range of the intermediate and large valve openings, such bubbles occur intermittently before the usual fixed cavitation appears on the plug surface. The string bubble is induced in a pressure-reducing section of a vortex core which extends from the valve inlet chamber to the outlet, and a vortex instability splits off a spherical bubble from the top end of the string in the process of the pressure recovery. Then a shock wave and a light "pop" sound are generated by the bubble collapse. The inception and the scale effect of valve size between 2 and 6 inches are discussed in a semiempirical manner by utilizing a vortex flow model. A practical method of suppressing the bubble generation is briefly mentioned.