[en] The Tokai Power Station, a 166 MWe gas cooled reactor, ceased commercial operation in March 31, 1998 and at present spent fuels are being discharged. Meanwhile, a specific plan for the decommissioning scheduled to be performed after the fuel discharging is being studied. The concrete waste of the station will account more than 90% of the total waste that will be caused by dismantling. Further, radioactive nuclides concentration of the concrete distributes widely from level of the low level radioactive waste (LLW) to below the clearance level. From these facts, it is important to evaluate precisely the radioactive concentration and amount of material corresponding to each concentration level in this area for the purpose of estimating dismantling process, man-hour required for dismantling work, dismantling cost, disposal cost of the waste from dismantling as well as exposure of radiation work employees and the public in the neighborhood of the plant. For this purpose, in a period from 1992 to 1998, core samples were taken by boring from typical positions of the primary and secondary biological shield and data have been collected about (1) the distribution of radioactive concentration of major radioactive nuclides in the biological shield wall along the concrete thickness and (2) the spacial distribution of radioactive concentration in the primary and secondary shields. In this paper, results of analysis of major radioactive nuclides in these concrete, and calculation results of radioactive nuclides generated by neutron irradiation are compared. (author)