  Gas accretion is a key process in star formation. However, the gas infall
detections in high-mass star forming regions with high-spatial resolution
observations are rare. Here we report the detection of gas infall towards a
cometary ultracompact H{\sc ii} region "C" in G34.26+0.15 complex. The hot core
associated with "C" has a mass of $\sim$76 M$_{\sun}$ and a volume density of
1.1$\times10^{8}$ cm$^{-3}$. The HCN (3--2), HCO$^{+}$ (1--0) lines observed by
single-dishes and the CN (2--1) lines observed by the SMA show redshifted
absorption features, indicating gas infall. We found a linear relationship
between the line width and optical depth of the CN (2--1) lines. Those
transitions with larger optical depth and line width have larger absorption
area. However, the infall velocities measured from different lines seem to be
constant, indicating the gas infall is uniform. We also investigated the
evolution of gas infall in high-mass star forming regions. At stages prior to
hot core phase, the typical infall velocity and mass infall rate are $\sim$ 1
km s$^{-1}$ and $\sim10^{-4}$ M$_{\sun}\cdot$yr$^{-1}$, respectively. While in
more evolved regions, the infall velocity and mass infall rates can reach as
high as serval km s$^{-1}$ and $\sim10^{-3}-10^{-2}$ M$_{\sun}\cdot$yr$^{-1}$,
respectively. Accelerated infall has been detected towards some hypercompact
H{\sc ii} and ultracompact H{\sc ii} regions. However, the acceleration
phenomenon becomes inapparent in more evolved ultracompact H{\sc ii} regions
(e.g. G34.26+0.15).
