Using water in high temperature and pressure, we produced hydrogen gas from cross-linked polyethylene (XLPE) efficiently. Around two moles of hydrogen gas were produced from a mole of carbon in XLPE at the optimum conditions of 700degreesC, 30 min, 20 of molar ratio of water to carbon in XLPE and 20 wt% nickel catalyst. This fact showed that hydrogen gas exceeding hydrogen in XLPE was produced by the reaction of plastic and water at high temperature and pressure. The production was accelerated by increasing the temperature and molar ratio of water to carbon in XLPE, but suppressed by increasing the pressure. Nickel and alkali catalysts gave excellent performance on the gas productivity from XLPE. Both catalysts promoted the degradation of plastic and water-gas shift reaction but did not affect the methanation much. The order of the catalytic activity was nickel > potassium hydroxide > sodium hydroxide. The equilibrium gas-phase composition calculated by MALT2 agreed with the experimental results, when XLPE was gasified completely.