Magnetic field generator useful for a magnetic resonance imaging instrument

A magnetic field generator comprises a permanent magnet, and pole pieces magnetically connected to the permanent magnet. The pole pieces are disposed to face each other to generate a magnetic field between them, and have a specific resistance of 20 μΩ-cm or more. The permanent magnet comprises a sintered alloy including iron as a main component, a rare earth element including yttrium, cobalt, and boron. The permanent magnet includes, as its main part, a strong magnetic Fe-rich phase of a tetragonal system, as well as a non-magnetic Laves phase, and preferably has a maximum energy product of 38 MGOe or more.