The concentration of radioactive cesium in sediments of Tokyo-bay, released by the Fukushima Dai-ichi nuclear power station accident, was measured every half year from July ’10 to February ’13 in order to analyze the trend of concentration. The samples were collected at two artificial deeper sites in dredged trenches and one natural shallower site, which were located off Makuhari in Tokyo-bay, then they were brought into a Ge detector to measure the γ-rays. According to an analysis of the upper layer of the samples, both 134Cs and 137Cs had been detected since the samples of August ’11, and they must have been released by the accident. Furthermore, from February ’12 to February ’13, the concentrations of 134Cs and 137Cs in upper layer of sediments had been higher at deeper sites than shallower site. The deeper sites look like pitfall traps for fine particles clinging to 134Cs and 137Cs, so we can call these sites "the hotspot in the sea". We also examined the depth profiles of 134Cs and 137Cs in samples taken on August ’12 and February ’13. As a result, 134Cs and 137Cs were found to have gone deeper in the sediment on February ’13 than on August ’12, and the inventory of them was also larger on February ’13. In addition, this phenomenon was observed more clearly at deeper sites than shallower site. Though 134Cs and 137Cs had not increased very much in upper layer from August ’12 to February ’13, we clarified that they had been flowing into the Tokyo-bay.
