NOVELTY - Process (I) involving reaction of magnesium chloride and water at 340degreesC or more is performed, and magnesium hydroxide, hydrogen chloride (HCl) are obtained. Process (II) involving reaction of ferrous chloride and water at 410degreesC or more is performed, and ferric hydroxide, HCl and hydrogen (H2) are formed. H2 is recovered and other products of processes (I) and (II) are subjected to subsequent process.    USE - For manufacture of hydrogen continuously by hydrothermal chemical decomposition using the heat ejected from chemical processing plant, iron manufacturing plant or power generating plant. The method can also be utilized for chemo-synthesis reaction such as fuel refining, desulfurization, semiconductor manufacture and iron ore reduction reactions. The hydrogen is also useful as fuel for fuel cell.    ADVANTAGE - Effective usage of waste heat eliminated from various plants which consumes abundant energy, is achieved. Hydrogen with high purity is obtained in large amount by the chemical thermolysis technique. The power supply of fuel cell can be promoted by supplying the hydrogen as fuel.    DETAILED DESCRIPTION - A method to manufacture hydrogen involves:    (i) Reacting magnesium chloride (MgCl2) and water (H2O) at 340degreesC or more and magnesium hydroxide (MgO), HCl are obtained;    (ii) Reacting ferrous chloride (FeCl2) and water at 410degreesC or more, and ferric hydroxide (Fe3O4), HCl and H2 are obtained. H2 is recovered and HCl obtained during steps (i) and (ii) is reacted with Fe3O4 obtained from step (ii), at 130degreesC or less and FeCl2, water and chlorine are formed. FeCl2 is subjected to (ii) step to form chlorine which is then reacted with MgO at 490degreesC or less. The products MgCl2 and oxygen are obtained. MgCl2 after removal of oxygen, is subjected to step (i). The water obtained during step (i) and (ii) are reused, and the above processes are repeated.    DESCRIPTION OF DRAWING(S) - The figure shows flow chart of hydrogen manufacturing process by hydrothermal chemical decomposition. (Drawing includes non-English language text)