Concentrations of ²³⁴U and ²³⁸U in the groundwater samples from Musashino-daichi, western Tokyo have been measured by alpha-spectrometry. Observed range of ²³⁴U and ²³⁸U concentrations were about 0.2-3.5 and 0.3-9.3 dpm/100l, respectively, with the ²³⁴U/²³⁸U activity ratios of 1.4-5.0 The presence of excess ²³⁴U, together with the observed linear correlation with ²³⁸U concentrations, indicates both excess ²³⁴U and²³⁸U have been supplied by source materials which are uniform with respect to the ability of supplying recoilgenic ²³⁴U and ²³⁸U into the groundwaters. High tritium concentrations in many of the samples (Tanaka, 1983) indicate the groundwater samples were composed of two distinct water components: one with fairly long age of contact with U containing minerals (orders of 1000 yrs or more) and one with fairly short tritium age (less than about 30 yrs).