**5. Conclusions**

In conclusion, the effect of the Coulomb interaction in interlayer magnetoresistance was elucidated in collaboration with theory and experiments for the Dirac electron system in organic conductor *α*-(BEDT-TTF)2I3. It was shown that enhancement of the effective *g*-factor due to the Coulomb interaction for *N* = 0 Landau levels depends on field angle *θ*. This was the key factor to explain the field-angle dependence of *B*0 at which interlayer resistance has a minimum.

The value of the effective *g*-factor obtained in the present study at *θ* = 0 under a strong magnetic field is consistent with the theoretical and experimental results of the Nernst effect [42,43]. This is also consistent with the theoretical and experimental results of the spin-lattice relaxation rate [33,36]. The quantum Hall ferromagnetic state assumed in the present study supports the helical surface state [29].

**Author Contributions:** T.T. and A.K. conceived and designed the theories; T.T. and A.K. performed the numerical calculations and analyzed the data; N.T. performed the experiments and analyzed the data; T.T., N.T., and A.K. wrote the paper.

**Funding:** This work was supported by MEXT/JSPJ KAKENHI under Grant Nos. 15K05166, 18K18739, and 16H06346.

**Acknowledgments:** The authors would like to thank S. Hayashi for the fruitful discussions.

**Conflicts of Interest:** The authors declare no conflict of interest.
