PURPOSE: To improve the electrical insulation between the electric poles of an MHD generator from the conventional example so that the generator can be operated for a long period by forming the split surfaces of insulating boards which are used for insulating the electric poles from each other and split in the peripheral direction to recessing and projecting surfaces and putting adjacent insulating boards together by the recessing and projecting surfaces.

CONSTITUTION: Side faces of split surfaces of insulating boards 8 are formed to recessing and projecting surfaces and adjacent insulating boards 8 are put together by the recessing and projecting surfaces. As a result, the electrical insulation is improved between the electric poles, since such a structure that the clearance between electric poles becomes longer, seed materials are hardly deposited continuously, and short circuit hardly occurs between electric poles is obtained.