An implicit finite-difference time-domain method (FDTD) using the principle of the alternating direction implicit (ADI) technique is introduced.The method is unconditionally stable

and the maximum time-step size is not limited by the Courant-Friedrich-Levy (CFL) condition

but rather by numerical errors.Compared with the conventional FDTD method

the time-step size of ADI-FDTD can be much larger

so the simulation time is shortened

especially when the grid size is much smaller than wavelength.The perfect matched layer (PML) technique is used in ADI-FDTD calculation for the first time.The expressions for the exponential time-stepping algorithm in PML are derived.The ADI-FDTD method is validated by an example.