Well water was contaminated by oil components in three houses living in K town located in the rural district in the northern part of Okayama prefecture in February of 1977. Authors conducted epidemiological survey and analysis of the well water. The results were obtained as follows. 1) Oil layer was recongized in the well erupted oil. Quantitative determination of oil components of well water obtained from two wells located in the neighbourhood was conducted. Data indicated that trace or no oil was detected by n-hexane extractive method and 0.3 ppm and 0.1 ppm of oil was detected by nondispersion type ultrared spectrophotometry. 2) It was recognized that oil components contaminating well water were derived from mineral oil by ultrared spectrophotometry of erupting oil. 3) Source of contamination was not clear. However, mineral oil-tanks and relational equipments in buildings with boilers or gasoline stands located in the neighbourhood of houses with contaminated well were considered. And it was suggested that mineral oil leaked from these sources and contaminated well water through underground. 4) It was confirmed that environmental pollution with mineral oil in such case could occur in rural district. Discussion was made that these evidence could be new problem in the field of public health.