Fallout radioactivity due to a leakage of the underground nuclear explosion test of U. S. S. R. (Jan. 15, 1965) was investigated. Abnormal increase of radioactive fallout was observed at first on the early morning of Jan. 20 at Nagaoka, Niigata Pref. In the fallout after Feb. 10, the influence by short lived nuclides could not be found. Gamma-ray spectrum of rain-snow sample of Jan. 20 coincided well with that of 140Ba + 140La, and the counting rate of gamma-rays decayed to a half in every 13 days through the beginning of March. Most part of gamma-activity was considered to be attributed to 140Ba + 140La in the transient equilibrium state. Radioactive product, 140Ba, was used as a checking source and 140Ba + 140La content in each fallout sample collected after Jan. 20 was estimated. After subtraction 140Ba + 140La activity from the total activity in each sample, the residual decayed almost exponentially, with a half-life of 51 ?? 53 days, extending for a fairly long period of time. Maximum beta-ray energy was estimated to be 1.5-1.6 MeV at the end of March when 140Ba + 140La was considered practically to have been extinct. Thus a chief component of the residual was thought to be 89Sr. Cumulative deposition of 140Ba + 140La and other nuclides during the period from Jan. 19 to Feb. 9 was estimated to be about 110 mCi/km2 and 270 mCi/km2 respectively as values of Feb. 15.