PURPOSE: To carry out the magnetic confinement of plasma without using a magnet by providing double helical coils in such a way that a discharge container is covered with a helical magnetic field and making a current flow in a reverse direction.

CONSTITUTION: Helical coils 9, 10 formed with double helical coils are set on the outside of a discharge container 2, and a current is made flow in the helical coils 9, 10 in the reverse direction, to form a helical magnetic field in a discharge room 8. As the wall surface of the discharge container 2 is covered with the helical magnetic field, electrons are caught by the helical magnetic field and moved along a line of magnetic force, reducing a zone of recombination with an Hg+ ion. Thereby, it is not necessary to use a magnet and the problem of a life due to a demagnetization characteristic is eliminated.