Seabed sediments were collected off the coast of Niigata Prefecture and Yamagata Prefecture and radioactivity was measured to identify the migration pathway of radioactive caesium from the Fukushima Dai-ichi Nuclear Power Plant. 134 Cs from the Fukushima Dai-ichi Nuclear Power Plant was detected in seabed sediment from offshore Sakata and Kamo (0.16±0.03 to 0.68±0.02 Bq/kg), but was not detected in seabed sediment from offshore Naoetsu. The results suggest that the potential source is the region northeast of Naoetsu to Sakata, including the Agano and Mogami Rivers.