  The Dirac electron in the organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$
under pressure is analyzed using a tight-binding model with nearest-neighbor
transfer energies and four molecules per unit cell. By noting that the Dirac
point between the first and second energy bands emerges or merges followed by
the level crossing at a time-reversal invariant momentum (TRIM), an effective
Hamiltonian is derived on the basis of these two wave functions at the TRIM,
which have different parities associated with an inversion symmetry around the
inversion center. We demonstrate that the Dirac point is determined by an
intersection of two kinds of lines originating from the Hamiltonian described
by symmetric and antisymmetric functions around the TRIM. The present method
quantitatively gives a reasonable location of the Dirac point of
$\alpha$-(BEDT-TTF)$_2$I$_3$ in a wide pressure range.
