In this paper, an error estimator is used to control the quality of fi
nite element solution in linear magnetostatic. This method, based on t
he works of P. Ladeveze, consists in constructing a complementary admi
ssible field from the one, calculated by the finite element method. Th
e quality of finite element solution is measured by comparing the coup
le of admissible solutions. This method has been developed in 2D in th
e case of scalar and vector potential formulations. To show the effect
iveness of this error estimator, we present three simple examples, one
of them having a known exact solution.