We propose an experimental method to examine the in-plane anisotropy of electronic structure in layered conductors. In the method, we measure the interlayer magnetoresistance as a function of in-plane magnetic field orientation. We applied it to an organic Dirac fermion system to experimentally determine the orientation of the anisotropic Dirac cones. It is concluded that the long axis of the elliptic constant-energy contours of the Dirac cone is tilted by approximately −30� from the crystalline axis to the axis under hydrostatic pressures. Additionally, we observed a signature of Van Hove singularity (which is a saddle point of the band dispersion) at 30–40 K above or below the Dirac point. The ridgeline of the saddle point is estimated as almost parallel to the crystalline axis.