A windowed internally circulating fluidized bed reactor was tested using -supported particles as redox material for thermochemical two-step water splitting to produce hydrogen from water. The internally circulating fluidized bed of particles is directly heated by solar-simulated Xe light irradiation through a transparent quartz window mounted on top of the reactor. A sun simulator with three Xe lamps at laboratory scale has been newly installed in our laboratory for testing the fluidized bed reactor. The input power of incident Xe light can be scaled up to . Temperature distributions within the fluidized bed are measured under concentrated Xe light irradiation with an input power of . Hydrogen productivity and reactivity for the fluidized bed of particles are examined using two different reactors under the flow rate and flow ratio, which yield a higher bed temperature. The feasibility of successive two-step water splitting using the fluidized bed reactor is examined by switching from gas flow in the thermal reduction step to a gas mixture in the water decomposition step. It is confirmed that hydrogen production takes place in the single fluidized bed reactor by successive two-step water splitting.