  We examine the magnetic susceptibility of topological insulators
microscopically and find that the orbital-Zeeman (OZ) cross term, the cross
term between the orbital effect and the spin Zeeman effect, is directly related
to the Berry curvature when the $z$-component of spin is conserved. In
particular, the OZ cross term reflects the spin Chern number, which results in
the quantization of the magnetic susceptibility jump at the topological phase
transition. The magnitude of the jump is in units of the universal value
$4|e|\mu_{\rm B}/h$. The physical origin of this quantization is clarified. We
also apply the obtained formula to an explicit model and demonstrate the
quantization.
