A terahertz (THz) waveguide using a metallic nanoslit whose width is much smaller than the skin depth is analytically investigated. By taking some important physical properties into account, we derive a simple, yet accurate, expression for the effective index. We also study the changes in modal field and the attenuation coefficient in the whole THz region, and find some interesting physical properties. Finally, we verify that these theoretical analyses coincide with the rigorous numerical simulations. This research can be useful for various applications of THz waveguides made of metallic nanoslits.