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Gas accretion is a key process in star formation. However, the gas infalldetections in high-mass star forming regions with high-spatial resolutionobservations are rare. Here we report the detection of gas infall towards acometary ultracompact H{\sc ii} region "C" in G34.26+0.15 complex. The hot coreassociated with "C" has a mass of $\sim$76 M$_{\sun}$ and a volume density of1.1$\times10^{8}$ cm$^{-3}$. The HCN (3--2), HCO$^{+}$ (1--0) lines observed bysingle-dishes and the CN (2--1) lines observed by the SMA show redshiftedabsorption features, indicating gas infall. We found a linear relationshipbetween the line width and optical depth of the CN (2--1) lines. Thosetransitions with larger optical depth and line width have larger absorptionarea. However, the infall velocities measured from different lines seem to beconstant, indicating the gas infall is uniform. We also investigated theevolution of gas infall in high-mass star forming regions. At stages prior tohot core phase, the typical infall velocity and mass infall rate are $\sim$ 1km s$^{-1}$ and $\sim10^{-4}$ M$_{\sun}\cdot$yr$^{-1}$, respectively. While inmore evolved regions, the infall velocity and mass infall rates can reach ashigh as serval km s$^{-1}$ and $\sim10^{-3}-10^{-2}$ M$_{\sun}\cdot$yr$^{-1}$,respectively. Accelerated infall has been detected towards some hypercompactH{\sc ii} and ultracompact H{\sc ii} regions. However, the accelerationphenomenon becomes inapparent in more evolved ultracompact H{\sc ii} regions(e.g. G34.26+0.15).

Uniform Infall toward the Cometary H II Region in the G34.26+0.15 Complex?