A new treatment for interface of metal and dielectric in finite-difference time domain (FDTD) method was presented, further how and why this treatment should be as it is was explained. The FDTD method was developed to compute the low-frequency electromagnetic fields in the metal instead of the traditional high-frequency ones as it was originally proposed. The computational results agreed well with the experimental ones. A new treatment for interface of metal and dielectric in finite-difference time domain (FDTD) method was presented, further how and why this treatment should be as it was was. The FDTD method was developed to compute the low-frequency electromagnetic fields in the metal instead of the traditional high-frequency ones as it was originally proposed. The computational results agreed well with the experimental ones.