  We report theoretical calculations of the diamagnetic susceptibility,
$\chi_0$, of helium atom. We determined the complete relativistic correction to
$\chi_0$ of the order of $\alpha^4$, where $\alpha$ is the fine structure
constant, by including all $\alpha^4$ terms originating from the Dirac and
Breit equations for a helium atom in a static magnetic field. Finite nuclear
mass corrections to $\chi_0$ was also evaluated. To obtain very accurate
results and reliable uncertainty estimates we used a sequence of explicitly
correlated basis sets of fully optimized Slater geminals. We found that
$\chi_0=-2.119\,106(34)\cdot10^{-5}$ $a_0^3$ and
$\chi_0=-2.119\,400(34)\cdot10^{-5}$ $a_0^3$ for $^4$He and $^3$He isotopes,
respectively, where $a_0$ is the Bohr radius and the uncertainties shown in the
parentheses are due entirely to the very conservative estimate of the neglected
QED corrections of the order of $\alpha^5$. Our results are compared with the
available experimental data and with previous, incomplete theoretical
determinations of the $\alpha^4$ contributions to the diamagnetic
susceptibility of helium.
