We performed optical-pump terahertz-probe measurements of a Mott insulator YTiO3 and a band semiconductor Si using a laser diode (1.47 eV) and a femtosecond-pulse laser (1.55 eV). Both samples possess long energy-relaxation times (1.5 ms for YTiO3 and 15 mu s for Si); therefore, it is possible to extract terahertz complex conductivities of photoinduced carriers under equilibrium. We observed highly contrasting behaviour-Drude conductivity in Si and localized conductivity possibly obeying the Jonscher law in YTiO3. The carrier number at the highest carrier-concentration layer in YTiO3 is estimated to be 0.015 per Ti site. Anisotropic conductivity of YTiO3 is determined. Our study indicates that localized carriers might play an important role in the incipient formation of photoinduced metallic phases in Mott insulators. In addition, this study shows that the transfer-matrix method is effective for extracting an optical constant of a sample with a spatially inhomogeneous carrier distribution.