[en] The method of estimating radionuclide composition of the wastes was carried out, in order to evaluate an inventory of those wastes contaminated by activated materials, which will come from the decommissioning of Tokai Nuclear Power Plant (GCR; 166 MW, shutdown in March 1998), owned by the Japan Atomic Power Company, and to plan the decommissioning of the plant. The approach to estimate the composition consists of specifying contamination sources, classifying the contaminated wastes into categories within which the composition is considered to be similar, and determining contribution degrees of the sources, based on measured values of the composition at every category. Four of fuel, fuel assembly (except fuels), carbon steel and graphite in the core are selected as contamination sources, and four of direct contaminated wastes, indirect contaminated wastes and gas filter in a cooling gas system, and all contaminated wastes in a liquid waste system are determined as categories. It was considered that this approach was proper by the comparison of the estimated compositions with the measured values. (author)