In the up-to-date diesel engine, boost pressure has been increased to improve the thermal efficiency as compared with a conventional diesel engine. High boost pressure results in a high gas density condition in the combustion chamber. Therefore, the mixing process of a diesel spray under high ambient gas density conditions may be different from that of a conventional diesel engine. In this study, particle image velocimetry (PIV) was applied to measurement of the velocity field inside a diesel spray under a high ambient gas density condition. From the results of PIV measurements, the maximum velocity gradient positions inside a spray were evaluated, and the effect of ambient gas density on the maximum velocity gradient position was investigated. It was found that the maximum velocity gradient position relative to the spray periphery shifted from the spray periphery to the center of the spray with increasing ambient gas density. A local equivalence ratio distribution inside the spray was calculated in accordance with momentum conservation and the spray velocity obtained with PIV. It was found that the region with sufficient air for combustion shifted upstream with increasing ambient gas density.