Reactive working material for use in hydrogen production by decomposition of water

A reaction medium comprising cubic zirconia and fine ferrite particles deposited thereon. It is for use in a process for producing hydrogen by utilizing solar heat, industrial waste heat, etc. to decompose water through a two-stage thermochemical water decomposition cycle. This reaction medium can prevent the fine ferrite particles from separating from the fine zirconia particles through volume fluctuations in repetitions of use. Thus, FeO particles as a reaction medium for the cyclic reaction conducted at a temperature as high as 1,400°C or above can be inhibited from growing through repetitions of melting and solidification.