  In Dirac semimetals, inter-band mixing has been known theoretically to give
rise to a giant orbital diamagnetism when the Fermi level is close to the Dirac
point. In Bi$ _{1-x}$Sb$ _x$ and other Dirac semimetals, an enhanced
diamagnetism in the magnetic susceptibility $\chi$ has been observed and
interpreted as a manifestation of such giant orbital diamagnetism.
Experimentally proving their orbital origin, however, has remained challenging.
Cubic antiperovskite Sr$ _3$PbO is a three-dimensional Dirac electron system
and shows the giant diamagnetism in $\chi$ as in the other Dirac semimetals. $
^{207}$Pb NMR measurements are conducted in this study to explore the
microscopic origin of diamagnetism. From the analysis of the Knight shift $K$
as a function of $\chi$ and the relaxation rate $T_1^{-1}$ for samples with
different hole densities, the spin and the orbital components in $K$ are
successfully separated. The results establish that the enhanced diamagnetism in
Sr$ _3$PbO originates from the orbital contribution of Dirac electrons, which
is fully consistent with the theory of giant orbital diamagnetism.
