  We investigated the complex conductivity spectrum of a Co-doped
BaFe$_2$As$_2$ epitaxial thin film in the THz region. In the normal state, the
complex conductivity shows a Drude-type frequency dependence, while in the
superconducting state, the frequency dependence of the complex conductivity
changes to that of a typical superconducting materials. We estimated the
magnetic penetration depth at absolute zero to be 710 nm and the
superconducting gap energy to be 2.8 meV, which is considered to be the
superconducting gap opened at the electron-type Fermi surface near the M point.
We succeeded in obtaining the low-energy elementary excitation of a Fe-based
superconductor using the electromagnetic method without invoking the
Kramers-Kronig transformation.
