The effect of the hole diameter on the surface wave ͑SW͒ excited on the metal surface perforated periodically with circular holes has been investigated in the terahertz ͑THz͒ region. We find that the SW is localized more strongly on the metal surface with increasing the hole diameter. These results provide the evidence for the existence of the SW-like mode, having properties similar to the surface plasmon polariton in the visible region, even in the THz or lower frequency regions, where metals are regarded as the nearly perfect conductor.The resonant light transmission phenomenon for metalhole arrays is attracting much attention with regard to underlying physics 1-5 and the development of potential applications. [6][7][8] The transmittance of the metal-hole array is several times higher than that expected from the hole aperture fraction. The mechanism of such an anomalous transmission characteristic is nowadays widely understood in terms of the excitation of the surface waves ͑SWs͒ 2-5,9 on the periodically structured metal surface and Fano-like interference between the SW and directly transmitted electromagnetic wave. 4 Here, we should empasize that in the 1960s similar resonant transmission characteristics of metal meshes were reported in the millimeter wave or far infrared regimes, 10-12 and the contribution of SWs, called Zenneck waves, was discussed in Ref. 10.The intensity of the SW is attenuated exponentially with distance from the metal-dielectric interface. The attenuation lengths both into the dielectric and metal sides are comparable to or shorter than the wavelength of light in the visible or near infrared regions. In the terahertz ͑THz͒ or lower frequency regions, on the other hand, the attenuation length of the SW into the dielectric side is several orders of magnitude longer than the wavelength for a flat metal surface, while the attenuation length into the metal side is several orders of magnitude shorter than the wavelength. In this case, one cannot recognize such a delocalized electromagnetic wave as the SW. Owing to the existence of the perforated structure, however, the attenuation length of the SW is possibly reduced to the order of the wavelength 13,14 and thus the resonant transmission phenomenon can be observed even in the THz and lower frequency regions. 15 Such SWs ͑called spoof surface plasmon polariton͒ have a possibility to cause a variety of optical characteristics of metal-hole arrays. In our previous study, for example, we observed the interesting transmission characteristics of double-layer metal-hole arrays based on the near-field coupling between the SWs excited on the two metal surfaces. 16 The surface localization property and lifetime of the SW on the metal-hole arrays depend on some geometrical parameters, such as the hole diameter, slab thickness, lattice constant, and permittivity of metals and are important for designing the optical devices made of metal-hole arrays. In this paper, we measured the hole diameter dependence of the surface-wave characteristics on the perfora...