A structure of a nuclear fusion reactor having a vacuum vessel in which hydrogen isotope plasma is enclosed and a confining magnetic field generating coil for confining said plasma at a predetermined position in said vacuum vessel. It comprises a low tritium-permeable layer having lower tritium-permeability than that of a cooling metal base for forming a refrigerant passage for cooling the vacuum vessel on at least the surface adjacent to said plasma enclosed and a heat resistant and insulating fire member of the level higher than that of said cooling metal base for thermally shielding said low tritium-permeable layer from said plasma or corpuscular rays is formed on the low tritium-permeable layer. The similar processings are applied to the cooling metal base for forming the refrigerant passage for cooling a divertor disposed in the vacuum vessel and for neutralizing ionized corpuscles so as to exhaust them.