  We theoretically investigate the interlayer magnetoresistance in
quasi-two-dimensional Dirac fermion systems, where the Fermi energy is at the
Dirac point. If there is an intermediate insulating layer that has an overlap
with the wave functions in the Dirac fermion layers, there appears a positive
magnetoresistance regime due to the intervalley tunneling. We show that the
interlayer magnetoresistance can be used to find whether Dirac fermions are
massive or not from the minimum in the interlayer magnetoresistance. As a
specific system, we consider \alphaI under high pressure. We also discuss that
one has to be careful in analyzing the crossover temperature from the positive
to negative magnetoresistance. A simple picture is applied to the crossover in
the zero temperature limit but it does not apply to the data at finite
temperatures. We show that the ratio of the Fermi velocity to the scattering
rate is evaluated from the zero temperature limit of the crossover temperature.
