[en] Recently, trace elements including rare earth elements have become to be used in large quantity for advanced industries. Those are expected to be carried into sea in the end. The concentration of those elements in water is very low, and the diurnal change of water quality is large, therefore, it is not easy to grasp the present state by the analysis of water samples. It has been known that aquatics concentrate the various trace compositions contained in water to high concentration. Also it is considered that aquatics undergo all the effects of water quality during their period of living. Attention has been paid to mussels as the index living thing for monitoring the heavy metals in seawater, and the monitoring of sea contamination has been attempted by analyzing them. As the technique for monitoring the state of contamination of Tokyo Bay with heavy metals and others, the analysis of many elements was attempted by the instrument neutron activation analysis of the mussels in Tokyo Bay. In this research, the factors that disperse the analysis values were mainly investigated. The samples and the analysis method and the results are reported. The variation of the concentration of elements in mussels can be decreased by the treatment of sample breeding and calculation. (K.I.)