The model reduction based on the method of snapshots is applied to the finite element analysis of three dimensional transient eddy current problems. It is known that accuracy of the reduced model highly depends on the number of snapshots. In this paper, we introduce a novel method which determines the adequate number of snapshots automatically. It is shown that the computational time can be reduced when using the model reduction based on the present method.Index Terms-Model order reduction, method of snapshots, proper orthogonal decomposition, finite element method, eddy current problem.