PURPOSE: To easily regenerate and decomtanimate, by melting a part of natrium by means of an induction coil located around a shell of a cold trap and by moving a melting area in such manner that moves the induction coil.

CONSTITUTION: A wire mesh 3 made of K S steel and the like is filled into a cold trap 1 equipped with a cylindrical shell 2, ends of which are formed round. The cold trap, where impurities are accumulated, is solidified by stopping an operation for purifying natrium. An induction coil 8 is located around the cold trap 1, and induction-heats partially the mesh 3 filled at the inside. At that time a part of natrium also is melted, and a strip melting area 9 is formed. Then, when the induction coil 8 is moved in the direction of an axis, the strip melting area 9 also moves togetherwith the coil, and impurities in natrium gather at a upper end or a bottom end of the shell 2. This permits natrium to be easily regenerated and decontaminated.