PURPOSE: To obtain a magnetic field generating device for magnetohydrodynamic power generation with a high generation efficiency by making uniform magnetic flux density distribution of a conductive fluid within a channel.

CONSTITUTION: The title device allows at least a pair of permanent magnets 5 and 7 to be provided on the inner-periphery surface of a yoke which is formed so that two sets of parallel sides exist on a cross section so that different poles oppose each other while sandwiching the channel of a conductive fluid. Then, a first permanent magnet 5 generating a main magnetic field is provided through a spacing each while sandwiching a plane including a center axis of a channel 4 of the conductive fluid. A second permanent magnet 7 generating a sub magnetic field is provided on the inner-periphery surface of a yoke 20 which crosses the yoke 1 where the first permanent magnet 5 is provided through a spacing each while sandwiching a plane crossing the plane and at the same time a magnetic pole at the side of the channel 4 of the second permanent magnet 7 is formed at the same pole as the magnetic pole of the first permanent magnet 5. Also, division plates 6 and 8 consisting of a non-magnetic material are provide at the spacing between the permanent magnets.