From　the　viewpoint　of　quasinormal　modes,　we　describe　a　decoherence　mechanism　of　charge　qubit　of　Josephson　junctions　(JJ)　in　a　lossy　microcavity,　which　can　appear　in　a　realistic　experiment　for　quantum　computation　based　on　a　JJ　qubit.　We　show　that　nonlinear　coupling　of　a　charge　qubit　to　the　quantum　cavity　field　can　result　in　additional　dissipation　of　the　resonant　mode　due　to　the　effective　interaction　between　those　nonresonant　modes　and　the　resonant　mode,　which　is　induced　by　the　charge　qubit　itself.　We　calculate　the　characteristic　time　of　the　decoherence　by　making　use　of　the　system　plus　bath　method.