By using the plane wave expansion method, we have calculated the band gap for a metallic slab with air rods in the transverse magnetic (TM) mode. The metallic structure of this system is an identical, symmetric structure and an infinite array of air rods. The arrangement of the air rods we used are a square lattice and a triangular lattice. The dielectric function of the metallic substrate is of the free electron form ε(ω)=1-(ω(superscript 2 subscript p)/ ω^2), where ω(subscript p) is the plasma frequency of the conducting electron. We present the band structure of the square lattice and triangular lattice.