A water treatment method sufficiently decomposes low-degradable organic substances while suppressing production of bromate ions and consumption of hydrogen peroxide. A water treatment method for purifying water using radicals includes injecting an electron-donating substance selected from the group consisting of saccharides, amino acids, lipids, humic acid, and mixtures thereof into water. The radicals are preferably produced using ozone and hydrogen peroxide in combination. Further, the electron-donating substance is preferably injected into the water in a concentration of 0.1 .mu.mol/L to 30 .mu.mol/L.