  Crystal field level scheme of a uniaxial chiral helimagnet YbNi$_3$Al$_9$,
exhibiting a chiral magnetic soliton lattice state by Cu substitution for Ni,
has been determined by inelastic neutron scattering. The ground and the first
excited doublets are separated by 44 K and are simply expressed as $\alpha|\pm
7/2\rangle + \beta |\mp 5/2\rangle$ with $\alpha$ and $\beta$ nearly equal to
$\pm 1/\sqrt{2}$. The easy axis of the crystal field anisotropy is the $c$ axis
when the excited levels are populated at high temperatures and high magnetic
fields. On the other hand, the magnetism at low temperatures and low magnetic
fields, where only the ground doublet is populated, is described by an easy
plane anisotropy which may be treated as an $S=1/2$ system with an anisotropic
$g$-factor, $g_{xy}=3.02$ and $g_z=1.14$. An orbital dependent exchange
interaction is also discussed to explain the temperature dependence of the
magnetic susceptibility based on this level scheme.
