  The ratio of the Zeeman splitting to the cyclotron energy ($M=\Delta E_Z /
\hbar \omega_c$), which characterizes the relative strength of the spin-orbit
interaction in crystals, is examined for the narrow gap IV-VI semiconductors
PbTe, SnTe, and their alloy Pb$_{1-x}$Sn$_x$Te on the basis of the multiband
$k\cdot p$ theory. The inverse mass $\alpha$, the g-factor $g$, and $M$ are
calculated numerically by employing the relativistic empirical tight-binding
band calculation. On the other hand, a simple but exact formula of $M$ is
obtained for the six-band model based on the group theoretical analysis. It is
shown that $M<1$ for PbTe and $M>1$ for SnTe, which are interpreted in terms of
the relevance of the interband couplings due to the crystalline spin-orbit
interaction. It is clarified both analytically and numerically that $M=1$ just
at the band inversion point, where the transition from trivial to nontrivial
topological crystalline insulator occurs. By using this property, one can
detect the transition point only with the bulk measurements. It is also
proposed that $M$ is useful to evaluate quantitatively a degree of the Dirac
electrons in solids.
