This paper presents the application of the stochastic finite element method to a magnetic field analysis. The dimensions or the material constants of a magnetic device are considered as probable ones and treated in the space-stochastic process. When magnetic permeability provides uncertain distribution on the surface or in the body of the magnetic material, the uncertainty can be taken into account. In order to obtain the effect of the deviation, a number of repeated calculations are necessary with the conventional finite element method. The present stochastic finite element method, however, needs no repetition, so that computing time can thus be considerably reduced. The calculated standard deviation makes it possible to evaluate the reliability of magnetic materials or power apparatus.