We experimentally and theoretically investigate the Gouy phase shift of the longitudinal terahertz (THz) wave in the entire focal region. By comparing the pulses shape and orientation before and after the focus, we find that the complete Gouy shift for the longitudinal field components is same to that of the transverse field components, with the value of π. The only different between them is their phase difference π/2. These results have important implications for the near-field optical microscopy and super-resolution imaging.