A two-dimensional photonic crystal slab having a photonic band gap common to a light beam in the TE-like mode and a light beam in the TM-like mode includes a slab member containing a high refractive index material and low refractive index sectors, arranged in the slab member in a triangular pattern, having a triangular prism shape. The two-dimensional photonic crystal slab further includes a linear defect section that is a disordered portion in a periodic structure of a photonic crystal and extends in the .GAMMA.-J direction. Light beams in the TE-like mode and light beams in the TM-like mode can be propagated through the waveguide section. A two-dimensional photonic crystal waveguide includes the two-dimensional photonic crystal slab.