A boil-off gas processing system for reliably burning a boil-off gas. The system processes a boil-off gas produced from a liquid hydrogen tank which is built in a hydrogen fueled vehicle. The system includes a mixing device for introducing air into a discharge passage through which the boil-off gas from the liquid hydrogen tank passes and for mixing the air and the boil-off gas and outputting a mixed gas; a catalytic combustor for burning the mixed gas which was mixed by the mixing device, the catalytic combustor having an inlet through which the mixed gas is introduced and an outlet for discharging combustion gas; an electric heater provided at the inlet side of the catalytic combustor; and a control section for controlling energizing of the electric heater.