  Dirac electrons in a single-component molecular conductor [Pd(dddt)$_2$]
under pressure have been examined using a tight-binding model which consists of
HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular
Orbital) functions in four molecules per unit cell. The Dirac cone between the
conduction and valence bands comes from the property that HOMO has ungerade
symmetry and LUMO has gerade symmetry. The Dirac point forms a loop in the
three-dimensional Brillouin zone, which is symmetric with respect to the plane
of $k_y$ = 0 where $k_y$ is the intralayer momentum along the molecular
stacking direction, i.e., with the largest (HOMO-HOMO, LUMO-LUMO) transfer
energy. The parity at TRIM (time reversal invariant momentum) is calculated
using the inversion symmetry around the lattice point of the crystal. It is
shown that such an exotic Dirac electron is understood from the parity of the
wave function at the TRIM and also from an effective Hamiltonian.
