  We discuss the excitation spectra around the Dirac node on a surface of a
three-dimensional topological insulator. By using the diagrammatic expansion,
we show that the coupling of an electron with the gauge field in the presence
of impurity scatterings opens a gap around the Dirac node. The results are
consistent with a recent experimental finding by T. Sato,et. al.[ Nature Phys.
\textbf{7}, 840 (2011)]. We also discuss the consistency between the present
results and the bulk-boundary correspondence of the topological insulator in
the presence of time reversal symmetry. The conclusion can be applied any two
dimensional massless fermions.
