PURPOSE:To seal a capsule consisting of W, etc., having the m. p. higher than the m. p. of a high melting nuclear fuel material such as nuclear fuel material without exerting a thermal influence upon the nuclear fuel material by forming the sealing part of the capsule and cap into a thin-walled construction and quickly welding the same with the smaller welding energy in the stage of sealing the above-mentioned nuclear fuel material into the above- mentioned capsule. CONSTITUTION:The high melting nuclear fuel material such as nuclear fuel material is put into the capsule body 10 made of W having the m. p. further than the m. p. of the nuclear fuel material and the cap 14 made of W is welded thereto to seal the nuclear fuel material into the capsule in the case of measuring the m. p. of the above-mentioned nuclear fuel mate rial by a heat detaining method of the capsule sealing type. A thin-walled opening edge 18 is provided to the opening at the top end of the above-mentioned cylindrical capsule 10 made of W and a thin-walled riser part 16 provided to the peripheral part of the cap 14 is fitted to the step part 20 thereof. The step part 24 is welded by an electron beam welding method. Since the two members 16, 18 in the fitting part are thin-walled, the quick welding is executed with the smaller welding energy and the nuclear fuel material can be sealed into the capsule with substantially no thermal influence of welding on the nuclear fuel material in the capsule.