After a contaminant reached the groundwater, we often notice the pollution of soil and/or the groundwater for the first time. When a pollutant reaches the groundwater, the pollutant spreads along with groundwater flow. Because the contamination spreads in wide areas, its removal becomes very difficult and time consuming. It is important to detect a trace of the pollutant in unsaturated soil before it reaches the groundwater. For the early detection of pollution, simple monitoring techniques have been desired. Therefore, the purpose of our research was to estimate the mineral-oil concentration in soil with a thermo-time domain reflectometry (thermo-TDR) technique. Soil used was a volcanic ash soil known as Kanto-loam. The soil was mixed with predetermined ratios of kerosene and distilled water for various contents of those liquids. We used a three-wire thermo-TDR probe (0.040m long, 0.0075m spacing, the center rod houses a nichrome heater wire and the outer two rods contain copper-constantan thermocouples) for measurements. Volumetric heat capacity was measured with a dual-probe heat-pulse technique, and was dielectric constant with TDR. The values of volumetric heat capacity and dielectric constant for mineral oil are dissimilar to those for other constituents. It is a promising sign for detecting oil in soil. We carried out this experiment to investigate the relationship among oil concentration, volumetric heat capacity, and dielectric constant. We estimate the oil content in the unsaturated soil from the results obtained. The results will be discussed.