Optimal coefficients of the special finite difference (FD) operator for the complex nonstandard FDTD (CNS-FDTD) method are presented. To derive the coefficients, we employ a semi-analytical method. The propagation constant is a complex number in lossy media, therefore our method is more complicated than the one for the NS-FDTD method. However, our proposed method is efficient for obtaining the optimal coefficients. It is confirmed by numerical tests that our coefficients give the CNS-FDTD method a higher accuracy than the FDTD method.