We investigate non-Hermitian Hamiltonian which governs system includes two-level atom and electromagnetical field. Using the notion of negativity, we study the degree of entanglement of a two- level atom interacting with a quantized electromagnetical field, described by the non-Hermitian Hamiltonian~\cite{kh1}. We assume that initially the filed is in a coherent pure state and the atom is in a general mixed state.
