PURPOSE: To prevent a thin glass tube from cracking, by melting an amalgam pellet in the thin glass tube below the straining point of the tube, and thereafter heating and sealing the tube to leave no strain therein.

CONSTITUTION: A rod 10 of the same material as a thin glass tube 7 and a columnar amalgam pellet 9 are put in the thin glass tube. The pellet 9 is then molten by a burner not shown in the drawing, so that the pellet is deformed to an ellipsoid whose minor diameter is nearly equal to the inside diameter of the thin glass tube 7. At that time, the temperature of the glass tube 7 at and near its portion in contact with the pellet 9 is lower than the straining point of the glass tube, so that no strain is left therein. The pellet 9 does not approach the portion of the tube 7 sealed by the burner, so that no strain is left in the sealed portion, either. The thin glass tube 7 is thus made unlikely to crack.