An optical element for terahertz waves that transmits terahertz-wave incident beams through itself after being compressed into half or less a wavelength of the beam, the optical element comprising: a substrate that allows terahertz waves to be transmitted through itself; and a conductive coat that covers a surface of the substrate and is capable of blocking the waves, wherein the coat has an aperture provided in a circular region and ring-shaped grooves, provided on a surface of the coat, having the same center as the circular region and having radii which increase every fixed length, and wherein in the circular region, four or more fan-shaped tips each having an arc with the same radius of curvature as the radius of the region, are arranged at equal distances toward the center of the circle without contacting each other as the arc contacts an outer periphery of the region.