  We report sub-pc-scale observations of the 321-GHz H$_2$O emission line in
the radio galaxy NGC 1052. The H$_2$O line emitter size is constrained in $<
0.6$ milliarcsec distributed on the continuum core component. The brightness
temperature exceeding $10^6$ K and the intensity variation indicate certain
evidence for maser emission. The maser spectrum consists of redshifted and
blueshifted velocity components spanning $\sim 400$ km s$^{-1}$, separated by a
local minimum around the systemic velocity of the galaxy. Spatial distribution
of maser components show velocity gradient along the jet direction, implying
that the population-inverted gas is driven by the jets interacting with the
molecular torus. We identified significant change of the maser spectra between
two sessions separated by 14 days. The maser profile showed a radial velocity
drift of $127 \pm 13$ km s$^{-1}$ yr$^{-1}$ implying inward gravitational
acceleration at 5000 Schwarzschild radii. The results demonstrate feasibility
of future VLBI observations to resolve the jet-torus interacting region.
