  We report multi-frequency and multi-epoch VLBI studies of the sub-parsec jet
in Sombrero galaxy (M 104, NGC 4594). Using Very Long Baseline Array data at
12, 22, 44, and 88 GHz, we study the kinematics of the jet and the properties
of the compact core. The sub-parsec jet is clearly detected at 12 and 22 GHz,
and the inner jet base is resolved down to $\sim70$ Schwarzschild radii
($R_{\rm s}$) at 44 GHz. The proper motions of the jet are measured with
apparent sub-relativistic speeds of $0.20\pm0.08 c$ and $0.05\pm0.02 c$ for the
approaching and the receding jet, respectively. Based on the apparent speed and
jet-to-counter-jet brightness ratio, we estimate the jet viewing angle to be
larger than $\sim37^{\circ}$, and the intrinsic speed to be between $\sim0.10
c$ and $0.40 c$. Their joint probability distribution suggests the most
probable values of the viewing angle and intrinsic speed to be
${66^{\circ}}^{+4^\circ}_{-6^\circ}$ and $0.19\pm0.04 c$, respectively. We also
find that the measured brightness temperatures of the core at 12, 22 and 44 GHz
are close to the equipartition brightness temperature, indicating that the
energy density of the radiating particles is comparable to the energy density
of the magnetic field in the sub-parsec jet region. Interestingly, the measured
core size at 88 GHz ($\sim25\pm5 R_{s}$) deviates from the expected frequency
dependence seen at lower frequencies. This may indicate a different origin for
the millimeter emission, which can explained by an Advection Dominated
Accretion Flow (ADAF) model. This model further predicts that at 230 and 340
GHz, the ADAF may dominate the radio emission over the jet.
