Steam generated by heating (80.degree. C.) pure water in a steam generating apparatus 1 is supplied into a reactor vessel 7 through a mass flow 6 from below. The reactor vessel 7 is filled with zeolite 8 which is granulated and to which metal halide is added. A heater 9 is provided around the reactor vessel 7. A conduit 10 is connected to the upper end of the reactor vessel 7. A bypass 11 for collecting a sample, a pressure gauge 12 and a N.sub.2 trap 13 are provided in the conduit 10. A suction pump 14 is connected to the N.sub.2 trap 13.