Introducing a chemical potential in the functional method, we construct the effective action of QED_3 with a Chern-Simons term. We examine a possibility that charge condensation 〈ψ^†ψ〉 remains nonzero at the limit of the zero chemical potential. If it happens, spontaneous magnetization occurs due to the Gauss' law constraint which connects the charge condensation to the background magnetic field. It is found that the stable vacuum with nonzero charge condensation is realized only when fermion

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... sses are sent to zero, keeping it lower than the chemical potential. This result suggests that the spontaneous magnetization is closely related to the fermion mass.