Gas-water ratio of the Osakabe wildcat was 5.9 at the production test. This value is Iarger than the gas solubilities in the completed formations. But, IPSs (Surface potential index of gas volume) of the Osakabe wildcat are smaller than the gas solubilities. It means that all of the gas in the new drilled hole did not enter the drilling fluid. So, we investigated flow mechanisms of formation and drilling fluids, when the production well “U-32” was drilled. The salinity concentration of the drilling fluid increases, because the formation water in the Southern Kanto gas field has so high salinity concentration as sea water. We measured the salinity concentration and calculated how much the formation water and gas were mixed into the drilling fluid.