The breakup and atomization processes of the pre-swirl spray, which is produced before the hollow-cone spray from a high-pressure swirl-type D.I. gasoline injector, were investigated under different ambient pressure conditions. The injector has a press-fitted swirl tip, in which six tangential slots giving the injecting fuel an angular momentum are perforated at an equal space interval. A microscopic imaging technique was applied to get the spatially high-resolution LIF tomograms of the pre-swirl spray. The sprays were illuminated by an Nd:YAG laser light sheet and imaged using a high resolution CCD camera, fixed with a micro lens and coupled with an optical low-pass filter. The droplet size and the individual droplet's velocity were obtained by applying the image processing and the particle tracking techniques, respectively. The mean droplet size, the non-circularity of the droplet and the droplet size-velocity correlations were investigated to clarify the breakup regimes of the pre-swirl spray. The breakup process of the pre-swirl spray shows an ambient pressure-dependence regime, and the breakup regime includes the following four stages in the order of changes: (a) liquid column stage (SOI-0.13 ms); (b) liquid blob stage (0.13-0.4 ms); (c) ligament stage for low ambient pressure and small-sized liquid blob stage for high ambient pressure (0.4-0.6 ms); and (d) large-sized droplet stage (0.6 ms-after). The stages (a), (b) and (c) belong to the primary breakup process, and the stage (d) belongs to the secondary breakup process. The SMD of the pre-swirl spray during the breakup of the liquid blob is very large with a wide scatter, and then it steeply decreases during the breakup processes of the long-ligaments and the small-sized liquid blobs. In addition, during the breakup process of the large-sized droplets (0.6-0.8 ms from SOI), the SMD steeply decreases with time from SOI, and it shows larger value at 0.1 MPa than at 0.4 MPa. This results from the increased aerodynamic forces as ambient pressure increases. The comparisons between the image processing and the laser diffraction based technique show that the laser diffraction based technique should be applied restrictively only at the longer axial distances where the pre-swirl spray mainly consists of the droplets within a measurable diameter range.