Base pressure distributions of an ACV over irregular ground are measured on two and three dimensional models. Furthermore, jet configurations of two dimensional model placed in water are observed by the aid of hydrogen bubble method. Remarkable results are as follows; Two dimensional model: (1) When both leading and trailing jets are Over higher portions or lower portions of the irregular ground, configurations of jets are as same as those on the plane ground in spite of unevenness in the cavity. (2) In case that one jet is on higher portion and the other jet is on lower portion of the irregular ground, this is underfed condition and that is overfed condition. Crossflow induced from overfed jet to underfed jet is depend upon the ratio of hover height to step height of the ground. (3) Overfed jet is accompanied with vortex attached to the base and underfed jet to the ground. Three dimensional model: (1) Though jet conditions are not so districtly corresponded to the geometrical shape of the ground, the parts of peripheral jet over higher portions are regarded as overfed conditions and other parts over lower portions are regarded as underfed conditions. (2) The intensities of overfed and underfed are affected by the distances from the boundary of the unevenness.