The beam power in a new project of a long-baseline neutrino oscillation experiment from Tokai to Kamioka (the T2K experiment) will be approximately 100 times higher than that in a substantial long-baseline neutrino oscillation experiment from the High Energy Accelerator Research Organization (KEK) to Kamioka (the K2K experiment). In the T2K experiment, radionuclides at serious activity levels will be produced in a target, magnetic horns, and partially dissolved into the cooling water. Radionuclides at serious activity levels will also be produced in the cooling water. Therefore, we measured the total activity and distribution of 7Be, 22Na, 54Mn, 57Co, 60Co, 88Y, 101gRh, 102mRh, and 110mAg collected on a demineralizer in the K2K water cooling system, in order to consider reducing both exposure to personnel from the demineralizer and radioactivity released by draining the cooling water at regular intervals in the T2K experiment. The total activity of the individual nuclides was estimated to range from 0.9 MBq to 0.7 GBq at the end of the 2-yr K2K operation. When the results are projected to the T2K experiment, 70 GBq of 7Be and 6 GBq of 22Na are particularly high, and shielding from the radiation must be provided for the entire water system. Half of the demineralizer was saturated with the Al, Cu, and Ag ions dissolved from the target and magnetic horns. When the entire column is saturated with the ions, all high activity of 22Na located at the bottom of the demineralizer will be released into the cooling water immediately. Although 88Y, 101gRh, and 102mRh cannot be collected completely because of weak retention by hydrolysis and/or association with colloid, a reduction in the surface area of the Ag metal is possible for new magnetic horns and will result in a decrease of the activities in the cooling water.