The tritium balance in a D-T fusion reactor is discussed in this paper comparing the amount of tritium consumed in the fueling cycle including the plasma vessel with the amount of tritium generated in the blanket system, using information reported so far. This comparison shows that the overall burning efficiency of tritium in the plasma vessel, the tritium loss ratio represented by tritium trapping in the redeposition layer of the plasma-facing material, and the recovery efficiency in the tritium breeding system play important roles in the tritium balance and that it may not be easy to maintain good tritium economy of a D-T fusion reactor if the proper combination of burning efficiency, tritium loss ratio, and tritium recovery efficiency is not obtained. The allowable limits for the overall tritium burning efficiency, for the tritium loss ratio in the fueling cycle, and for the recovery efficiency to secure the self-sustainable tritium system are also discussed in this paper.