The steam gasification of Taiheiyo coal char was carried out to determine gasification kinetics taking account of the concentration dependence of coexisting gases of H-2, CO2, CO and O-2. Gasification tests were conducted with five different gas mixtures as gasifying agents at atmospheric pressure and over a temperature range of 1073 to 1273 K using a differential fixed bed reactor. The reaction rate equation was expressed by such functions as temperature, partial pressure of the gases, changes of pore structure and surface area of char and the unconverted carbon fraction of the char. The dependence of the gasification rate on the partial pressure of H2O, CO2 and CO was able to be expressed by Langmuir-Hinshelwood kinetics which accounts for the decrease in the rate due to coexistence of H-2 and CO. The partial pressure of CO2 had little effect on the rate in the H2O-CO2 gasification because the reaction rate of H2O is greater than CO2. It was seen that the reaction rate of the char with O-2 depends on the diffusion coefficient of O-2 in the mixed gas. The rate equation based on the shrinking sphere model was in good agreement with the experimental result. The dependence of the steam reaction rate on the unconverted carbon fraction of partially burned char is almost the same as that of the char before burning.