The behaviors of unconfied groundwater and saline intrusion in the estuary basin of the Yoshino River are investigated by computer simulation method based on observations. The objective region is nearly rectangular with an area of about 50km². The monthly field survey of the groundwater levels and the chlorine-ion concertrations in 31 wells was continued for about 3 years from October, 1983. The contents of this study are 1) the hydraulic and stochastic analysis of the groundwater levels, 2) the identification of aquifer constants and the simulation of water balace based on the Tyson-Wever's equation, 3) the flow simulation by FEM and 4) the time-dependent analysis of the saline intrusion by FEM. The behabiors of groundwater and its salinity were estimatedin the quasi-three dimensional analysis. The most suitable aquifer constant, T=986.2m²/day, was calculated and the present state of groundwater flow was reproduced with good agreement. The salinity movement was discussed by using the FEM.