Gouy phase shift is a well-known behavior that occurs when a propagating light is focused, but its behavior in the sub-wavelength confinement is not yet known. Here, we report the theoretical and experimental study of the aperture-size dependency of the Gouy phase shift in the sub-wavelength diffraction regime. In experiments carried out with laser-induced terahertz (THz) wave emission from various semiconductor apertures, we demonstrate the use of Guoy phase shit for sub-wavelength THz microscopy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4705294]