Colrbicula japonica is a typical brackish bivalve and plays an important role in the brackish lake ecosystems. Large amount death of C. japonica is a very serious problem for both the fishery productivity and the water quality of brackish lakes. To make ways to avoid the large amount death, it is required to establish some values to check the condition of the bivalve. To clarify the value of the elements effective to check the condition of C. japonica, some experiments were carried out in this study. Seasonal changes of Condition Index defined as an index representing the content richness of the shell ( = A.F.D.W. / (shell length × shell height × shell width) × 1000), filtration rate and tolerance of anoxic condition, were examined for C. japonica living in an artificial lagoon built beside the Lake Jinzai, Shimane Prefecture, from March 2000 to January 2001. Although the Condition Index reduced largely from May to September, the filtration rate and the tolerance of anoxic condition did not reduce so apparently. It indicates that the artificial lagoon was kept in a comfortable condition for C. japonica through the experiment period. The Condition Index of C. japonica kept in an anoxic condition artificially reduced rapidly. It appears that the large amount death occurs when the reducing rate of the Condition Index becomes larger than 3%/day. In these cases of large amount death, the filtration rate is reduced to under 0.1 L/ind.・hr. The combination of these two values may provide a guide to check the condition of C. japonica and effective to predict the large amount death.