PURPOSE: To improve the parallelism and uniformity of lines of magnetic force in the inner peripheral area by joining integrally an inner and an outer magnets mutually different in magnetic characteristics to constitute an annular permanent magnet, and recessing the boundary line of connection to the side of center axis.

CONSTITUTION: The annular permanent magnet 10 used for the magnetic field generating device of the dry process apparatus utilizing magnetron discharge is formed by joining two kinds of magnets having different magnetic performances into one integrated structure. A 1st magnet 12 is disposed on the outer peripheral side of annular shape, and a 2nd magnet 14 on the side of the center axis O, and the connecting boundary line is preferably the ellipse in which the short diameter R3 is equal to the inner diameter (r) of the permanent magnet 10 and the long diameter R1 to the outer diameter R. The 1st magnet 12 is preferably made of the material of high coercive force and of low residual magnetic flux density, and the 2nd magnet 14 is preferably formed of low coercive force and of high residual magnetic flux density. By this method, uniform and parallel magnetic lines ar formed in the wide area contg. the inner peripheral part of the annular magnet. By using this magnetic field generating device, uniform dry process is realized.