Extensive field measurement of groundwater level and quality was carried out three times in 2002 at sixty five stations in Takamatsu area to clarify the effects of irrigation on the unconfined groundwater. Continuous measurement of groundwater level has also been made at seven stations in addition to the continuous measurement by the public sectors. It is found that the water level gradually increases in the irrigation period with the contours of water level running nearly parallel to those of topography. The rainfall and the pumping up have influence of the change in water level. The pH of groundwater is found to slightly increase in the irrigation season while Stiff diagrams and the cluster analysis demonstrate that the water quality can be classified into four groups and that the groundwater quality mostly remains unchanged. Nitrate nitrogen of unconfined water at a few locations has exceeded the environmental standard, which may have come from the overuse of fertilizer. Some wells in the coastal region have shown the effects of seawater intrusion.