In a nozzle hole of a diesel fuel injector, string cavitation may appear along the nozzle-hole axis which affects the fuel spray characteristics. Previous studies concluded that string cavitation is the result of the complex swirling flow in the upstream region. However, the mechanism of the swirling flow formation, the condition of string cavitation appearance, and its effects on the injected liquid jet are not well understood. In this study, visualization of cavitation in a cylindrical nozzle hole with an asymmetric inflow from various simple rectangular upstream channels was conducted. The effects of needle lift and the number of nozzle holes were examined by changing the height and the width of the upstream channel, respectively. High-speed images of string cavitation formation were captured. A simple numerical simulation was carried out to qualitatively examine the velocity and pressure distributions. As a result, we clarify the condition of string cavitation formation and propose the string cavitation regime maps which take dimensionless needle lift and liquid velocity as major parameters. We find that string cavitation and a resulting swirling hollow-cone liquid skirt are formed at low needle lift and high liquid velocity, while at high needle lift no string cavitation is formed. Under the narrower upstream channel width condition, which represents a fuel injector with many nozzle holes, string cavitation occurs only at much lower needle lift and higher liquid velocity. The numerical results show a good agreement with the experiment results; i.e., a slanted swirling inflow with low pressure along its axis is formed in the nozzle hole only under the low needle lift condition.