  Using a tight-binding model, we theoretically examine the anisotropic
conductivity of the nodal line semimetal of a three-dimensional Dirac electron
in a single-component molecular conductor [Pd(dddt)$_2$], which consists of
four molecules with HOMO and LUMO orbitals per unit cell. The conductivity
shows an anisotropy given by $\sigma_y > \sigma_x > \sigma_z$ in accordance
with that of the velocity of the Dirac cone where $z$ is the interlayer
direction and $y$ is the molecular stacking direction. With increasing
pressure, the nodal line semimetal emerges, followed by a loop of the Dirac
point where $\sigma_x$ takes its maximum at a pressure. Such a pressure
dependence is studied by calculating the density of states and chemical
potential. The temperature dependence of anisotropic conductivity is examined
at low temperatures to obtain a constant behavior, which is ascribed to the
Dirac electron. The relevance of the present calculation to the experiment is
discussed.
