PURPOSE: To strengthen the support for a coil by engaging a vacuum container and a troidal magnetic field coil by a projection and a recess formed on both, respectively.

CONSTITUTION: A projection 10 in the troidal direction is formed at a part on the troidal magnetic field coil 3 side of a vacuum container 2, and the coil 3 consists of a coil case 9 and a coil conductor 8, and a recess 11 in the troidal direction is formed in the whole region on the container 2 side of the case 9. The projection 10 of the container 2 and the recess 11 of the case 9 are fit-engaged through an electric insulating layer 12. Since the container 2 is firmly supported from a base, etc., by supporting legs, the displacement in the troidal direction of the coil 3 by a turning-down force can be prevented by this fitting engagement.