  The electronic structure of weak charge-ordering (CO) state just below the
critical pressure in an organic conductor \alpha-(BEDT-TTF)_2I_3 was
experimentally investigated using peak structure in the temperature dependence
of interlayer magnetoresistance (MR). Based on a minimal model considering
multiple Landau levels (LLs), we discuss herein the MR peak as characteristic
to multilayer massless/massive Dirac fermion (DF) systems. MR measured in the
weak CO state exhibited a clear MR peak, and its magnetic-field dependence was
consistent with the LL behavior of a massive DF with a small gap. Results
indicate that the weak CO state in \alpha-(BEDT-TTF)_2I_3 is a massive DF
state.
