1) The radioactivity derived from fission products in the water of rain and the river Ashida has been surveyed, and the radioactivity was newly found out in the water of that river at every rainfall from Dec. 1956 to Feb. 1957. During the rainfall, radioactivity was always detected in the water of the river Ashida.
2) When the rain stopped, the radioactivity in the river water began to decrease gradually and could not be found out after two days, and so on hereafter.
3) The radiological contamination of the brackish-water at the mouth of the river Ashida and also that of the sea water at the stations off the mouth of that river, at a few stations in the Seto Inland Sea and at the station in the Strait of Hoyo were surveyed respectively, and any remarkable radioactivity could not be found out in either case.
4) The radiological contamination of laver, cultured at the mouth of the river Ashida,
has not been recognized from Jan. to Feb. 1957 even when the radioactivity was detected
in the river water at every rainfall. During that term the values for the amounts of radioactivity in the river water was (0.41 ± 0.11) × 10-4 μc/l at the maximum value.

this work is indebted to the Ministry of Education for the research grant