Nitrate−N pollution has been widely observed in ground water of Japan which was caused by agricultural practice and increasing amounts of domestic wastes．This paper describes present situation of groundwater of the Yumigahama sandbar，San'in district，Japan．The results show severe contamination of nitrate-N and arsenic occurrences in many tubewells suggesting variation of the redox condition within aquifers．Arsenic solution has been inferred to occur under reducing condition which is not concordant to the oxic condition for the nitrate-N occurrences．High COD values are characteristic in groundwaters of arsenic contamination which may be related to bacterial deterioration of organic matter．This fermentation results in release of arsenic species from sediments and in nitrate reduction．Iron oxyhydroxides（FeOOH）adsorbing As species are not necessary for this reaction，thus iron was not always detected in arsenic contaminated water in Yumigahama．