The present paper focused on the chlorine distribution during RDF combustion in fluidized beds. To discuss the captured mechanism of chlorine by bottom ash at relatively high temperature (above 1,000 K) and water vapor concentration (about 10%) in which this phenomenon thermodynamically did not occur, the structure and chemical composition distribution on the surface and cross section of bed materials and bottom ash after RDF combustion in fluidized bed at 1,073 K were observed by SEM and EPMA. The thin layer of several mu m thickness with Ca and Si were formed on the surface of bed materials and bottom ash, and chlorine was captured in this layer. Furthermore, by the fundamental gas / solid reaction test of the hydrogen chloride inclusion gas (1,000 ppm) with the different water vapor concentration through CaO fixed bed and, it was confirmed that the amount of captured hydrogen chloride by CaO surpassed a thermodynamic equilibrium calculation result on condition of relatively high water vapor concentration up to 7.9%.