  This study investigates the spin susceptibility in U-based superconductor
UTe$_2$ in the superconducting (SC) state by using Knight shift measurements
for a magnetic field $H$ along the $a$ axis, which is the magnetic easy axis of
UTe$_2$. Although a tiny anomaly ascribed to the SC diamagnetic effect was
observed just below the SC transition temperature $T_{\rm c}$, the $a$-axis
Knight shift in the SC state shows no significant decrease, following the
extrapolation from the normal-state temperature dependence. This indicates that
the spin susceptibility is nearly unchanged below $T_{\rm c}$. Considering the
previous Knight shift results for $H \parallel b$ and $H \parallel c$, the
dominant SC state is determined to be $B_{\rm 3u}$ in the spin-triplet pairing,
which is consistent with the spin anisotropy in the normal state. The present
result shows that UTe$_2$ is a spin-triplet superconductor with spin degrees of
freedom.
