In order to develop a slagging coal gasifier and an Integral Gasification Combined Cycle (IGCC), fundamental behavior of combustion/gasification of a coal particle trapped on the molten slag layer was studied by using a laboratory stale batch reactor. Air and CO2 were used as reactants to burn and gasify the coal particle, respectively. The experiments were carried out by dropping a single coal particle onto the molten slag layer. The producted gas concentrations and the surface temperatures of coal particle and slag layer were continuously measured by gas analyzers and a two-dimensional two-color pyrometer, respectively. The indicated results by the analyzers were deconvoluted to evaluate the true time history of concentration change since the changing rate of concentration corresponded to the time delay of the analyzers comparably. In order to observe the change of particle structure, the reacting particle was quenched by injecting N-2 gas. After that, the surface and internal structures were observed by an SEM and an optical microscope, respectively. As a result, the accuracy of adopted temperature measuring method was found to be within +/-40 K. Under the combustion condition the particle temperature of coal exceeded that of the slag layer. Under the gasification condition, on the other hand, the reverse tendency was observed. The molten slag layer decelerated the reaction rate, especially under the gasification condition, since the melting points of ash and coal particle itself fell down due to the reducing condition occurring around/in the coal particle.