  We investigate Dirac fermions on the surface of the topological insulator
Bi2Se3 using scanning tunneling spectroscopy. Landau levels (LLs) are observed
in the tunneling spectra in a magnetic field. In contrast to LLs of
conventional electrons, a field independent LL appears at the Dirac point,
which is a hallmark of Dirac fermions. A scaling analysis of LLs based on the
Bohr-Sommerfeld quantization condition allowed us to determine the dispersion
of the surface band. Near the Fermi energy, fine peaks mixed with LLs appear in
the spectra, which may be responsible for the anomalous magneto-fingerprint
effect [J. G. Checkelsky et al., Phys. Rev. Lett. 103, 246601 (2009)].
