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A new design of fuel injector for diesel engines is expected to enhance fuel spray atomization at the end of the fuel injection for the reduction in particulate matter emission. It was pointed out that string cavitation may occur in a multi-hole mini-sac injector and increase spray angle. However, the complicated velocity distributions in the injectors have not been understood yet, which make it difficult for us to reveal the overall string cavitation phenomena. In this study, visualization of string cavitation in a transparent three-hole mini-sac injector and spray at various needle seat gaps and particle image velocimetry (PIV) analysis of the flow in a plane perpendicular to the axis of an orifice are carried out to clarify the cavitation and flow structure as well as spray characteristics. The results clarify that (1) at the low needle lift of Z/D = 1 twin string cavitation flow appears intermittently and (2) at the very low needle lift of Z/D = 0.5 single string cavitation flow with a hollow-cone spray occurs, which significantly increases spray angle.