  In the viewpoint of quasi normal modes, we describe a novel decoherence
mechanism of charge qubit of Josephson Junctions (JJ) in a lossy micro-cavity,
which can appear in the realistic experiment for quantum computation based on
JJ qubit. We show that the nonlinear coupling of a charge qubit to quantum
cavity field can result in an additional dissipation of resonant mode due to
its effective interaction between those non-resonant modes and a resonant mode,
which is induced by the charge qubit itself. We calculate the characterized
time of the novel decoherence by making use of the system plus bath method.
