  We present the theory of the inplane magnetoresistance in two-dimensional
massless Dirac fermion systems including the Zeeman splitting and the
electron-electron interaction effect on the Landau level broadening within a
random phase approximation. With the decrease in temperature, we find a
characteristic temperature dependence of the inplane magnetoresistance showing
a minimum followed by an enhancement with a plateau. The theory is in good
agreement with the experiment of the layered organic conductor
\alpha-(BEDT-TTF)_2I_3 under pressure. In-plane magnetoresistsnce of graphene
is also discussed based on this theory.
