Provided is a dipole ring magnetic field generator capable of generating a substantially unidirectional magnetic field in the internal space of a ring without using permanent magnet pieces having fan-shaped or trapezoidal sections, and as a result, a smaller skew angle therein is achieved. The sections of the permanent magnet pieces are shaped to be rectangular, and a plurality of the rectangular permanent magnet pieces are circularly placed at predetermined positions. Each of first, second, third and fourth permanent magnet units, which are main permanent magnet units, comprises five or more permanent magnet pieces. The first and third permanent magnet units, as well as the second and fourth permanent magnet units, are positioned oppositely from each other with respect to a central axis of the magnetic field generator and have hollow shapes or E-shapes facing against each other in a section perpendicular to the central axis.