Interest continues to grow in controlling the propagation of electromagnetic waves by utilizing periodically or randomly arranged artificial structures made of metal, dielectric, and other materials. When the size of the constituent structures and the separation between the neighboring structures are much smaller than the wavelength of the electromagnetic waves, the structure arrays behave as a continuous medium for the electromagnetic waves. That is, macroscopic medium parameters such as effective permittivity and permeability can be defined for the array. The artificial continuous medium is called a "metamaterial." In the frequency region below the microwave frequency, the use of metallic structures as artificial media has been studied since the late 1940's (Collin, 1990). At first, only control of the permittivity was studied and not that of the permeability. However, Pendry et al. (1999) proposed methods for fabricating artificial magnetic media, namely, magnetic metamaterials, which were built from nonmagnetic conductors. It was shown that not only can relative permeability be changed from unity but it also can have a negative value. Although the relative permeabilities of naturally occurring media are almost unity in such high frequency regions as microwave, terahertz, and optical regions, the restriction that the relative permeability is almost unity can be removed using the metamaterial. Moreover, the magnetic metamaterial enabled us to fabricate media with simultaneous negative permittivity and permeability, or negative refractive index media that were predicted by Veselago (1968). In fact, Shelby et al. (2001) made the first experimental verification of a negative refractive index metamaterial in the microwave region. This increased researcher interest in metamaterials. It was not possible to independently control the wavenumber and the wave impedance in a medium until magnetic metamaterials were developed. The wavenumber is related to the propagation and refraction of electromagnetic waves, and the wave impedance is connected with the reflection. Phenomena about electromagnetic waves are described by these two quantities. In dielectric media, both of the wavenumber and wave impedance change with a change of the permittivity, and we cannot set these parameters independently. However, the wavenumber and wave impedance can be changed independently in metamaterials because we can control the permeability as well as the permittivity with metamaterials. By utilizing the flexibility of the wavenumber and wave impedance in metamaterials, such novel phenomena as a perfect lens (superlens) (