PURPOSE: To permit an MHD generator to be easily increased in size, by retaining a pair of circular coils of a superdonductor such that they are effectively supported in relation to the attractive force acting therebetween and are maintained at a predetermined temperature.

CONSTITUTION: A helium vessel 12 and a vacuum vessel 17 are added for maintaining the properties of a circular coil 11. Ring-shaped support members 13, 14, 15, etc. are provided which effectively support the helium vessel 12 in relation to the attractive force acting between the circular coils 11 disposed on the respecitive sides of disks 1b, 1c, while maintaining heat conduction. Inside the vacuum vessel 17, the lower end portion of the support ring 15 abuts against the inner wall portion of the vacuum vessel 17 and supports the support ring 14. On the other hand, the helium vessel 12 is supported by the intermediate support ring 14 through the support ring 13. The helium vessel 12 contacts with the vacuum vessel 17 only through an FRP member of excellent heat insulating properties. Therefore, both the vessels are thermally insulated from each other, so that each circular coil 11 is maintained at a predetermined temperature.