We investigated the magnetoresistance and the Hall effect in an organic massless Dirac fermion system α-(BETS) 2 I 3 under pressure. The Fermi energy of this system is slightly far away from the Dirac points, and thus the ν = 1 quantum Hall state is realized in a low magnetic field at low temperatures. Moreover, the experimental formula for chemical potential as a function of temperature is clarified. We succeeded in detecting the inter-band effects of the magnetic field on the Hall conductivity when the chemical potential passes the Dirac points.The discovery of graphene 1, 2) opened up a new field of "massless Dirac fermion" in condensed matter physics. A wide variety of materials with massless Dirac fermions have been now established.The massless Dirac fermion system has a unique energy structure with two cone-type bands touching each other at discrete Dirac points in the first Brillouin zone. The low-energy excitations described by a relativistic Dirac equation give rise to the π Berry phase that characterizes the transport of the Dirac fermions.In such a system, we are interested in how the Dirac point becomes unstable and creates a mass gap, e.g., through electron-electron interactions or spin-orbit coupling.To address this question, we should target Dirac materials in which the electron-electron interaction can be controlled and the Fermi energy is in the Dirac points. To our knowledge, only organic massless Dirac fermion systems can be responded those requests.Among them, α-(BEDT-TTF) 2 I 33) undergoes a charge-order phase transition at 135 K under ambient pressure, [4][5][6][7][8][9] and a massless Dirac fermion system is realized at pressures above 1.5 GPa. [10][11][12][13][14][15][16][17][18][19] This Dirac fermion system undergoes a crossover from a two-dimensional (2D)