Disclosed is an optical delay element that makes use of a line-defect waveguide of a photonic crystal, in which long optical delay time and small group speed dispersion are rendered compatible with each other and in which waveform distortion that might otherwise be produced in processing an ultra-high speed signal is eliminated. Two line-defect waveguides 5 and 11, having different pillar diameters and group velocity dispersions of opposite signs, are interconnected by a line-defect waveguide 8, the pillar diameters of which are gradually varied from one 5 of the line-defect waveguides to the other line-defect waveguide 11, such as to compensate for group speed dispersion as well as to maintain an optical delay effect.