  Optical excitations of BiTeI with large Rashba spin splitting have been
studied in an external magnetic field ($B$) applied parallel to the polar axis.
A sequence of transitions between the Landau levels (LLs), whose energies are
in proportion to $\sqrt{B}$ were observed, being characteristic of massless
Dirac electrons. The large separation energy between the LLs makes it possible
to detect the strongest cyclotron resonance even at room temperature in
moderate fields. Unlike in 2D Dirac systems, the magnetic field induced
rearrangement of the conductivity spectrum is directly observed.
