  By deriving a tight-binding model, we demonstrate a mechanism of forming a
nodal line of Dirac points in a single-component molecular conductor
[Pt(dmtd)$_2$] [Zhou {\it et al.}, Chem. Commun. {\bfseries 55}, 3327 (2019)],
consisting of HOMO and LUMO. The nodal line is obtained as the intersection of
two surfaces, where one corresponds to the HOMO-LUMO band crossing and another
is vanishing of the HOMO-LUMO couplings due to their different symmetries. The
latter property is essential for the Dirac electron in molecular conductors.
The nature of the open nodal line is discussed in terms of the parity of the
wavefunctions at eight TRIMs (time reversal invariant momenta).
