Guided modes in a two-dimensional photonic crystal consisting of nearly-free-electron metals are considered. To avoid time-consuming convolution, modified time-stepping formulae are used in a finite-difference time-domain approach. The guided modes in the metallic photonic crystal waveguide are related to those in a conventional metallic waveguide. A cut-off frequency exists, and consequently a mode gap at low frequencies exists in the photonic crystal metallic waveguide.