Stoichiometry of the steam reforming of methane (CH4 + H2O = CO + 3H(2)) was studied using a steam generator, which can keep the steam content in the reactant gases (methane and steam). The reactor was irradiated using the concentrated Xe-lamp beam (solar simulator) to 650 degrees C. The mole ratio of H2O/CH4 in the reactant gases was kept 1/1, and the Ni/Al2O3 catalyst was used. The deposited C and unreacted H2O amounts were calculated by the mass balance equations with the assumption that inlet gases of CH4 + H2O produce outlet gases of CO, CO2, H-2, CH4 and H2O, and Carbon. The gaseous contents in the outlet gases from the reactor were estimated from the data of the gas chromatography. Those values and the calculated C and H2O values were close to those of equilibrium condition theoretically evaluated using thermodynamic data at 650 degrees C. The inlet gases controlled at the mole ratio of H2O/CH4 = 1/1 converted to CO and CO2 (CO/CO2 = 3/1) stoichiometrically on the Ni/Al2O3 catalyst at 650 degrees C.