The effects of the amount of adsorbed trichloroethylene (TCE) and temperature on the photocatalytic decomposition of TCE in an oxygen-based gas were investigated using TiO2 catalyst supported on silica gel beads (TiO2/SiO2 bead catalyst). Experiments were carried out in the temperature range of 303-353 K and TCE concentration range of 50-500 ppm. The effects were evaluated quantitatively in terms of the decomposition ratio of TCE, defined by the ratio of the decomposed TCE amount to the adsorbed TCE amount, and the fractions Of CO2 and HCl products in the decomposed TCE. From the dependence of the TCE photocatalytic decomposition on the initial amount of adsorbed TCE at a constant temperature of 303 K, the optimum amount of adsorbed TCE on the TiO2/SiO2 catalyst for effective decomposition was found to be 8.0-15 mmol. In this range, the decomposition ratio of TCE, and the fractions of HCl and CO2 were respectively about 85, 70 and 60%. As for the effects of reaction temperature on the photocatalytic decomposition, the decomposition ratio of TCE was independent of temperature within experimental error, but the fractions Of CO2 and HCl in the decomposed TCE increased as temperature decreased.