  Whilst the g-factor can be anisotropic due to the spin-orbit interaction
(SOI), its existence in solids cannot be simply asserted from a band structure,
which hinders progress on studies from such the viewpoints. The g-factor in
bismuth (Bi) is largely anisotropic; especially for holes at T-point, the
g-factor perpendicular to the trigonal axis is negligibly small (< 0.112),
whereas the g-factor along the trigonal axis is very large (62.7). We clarified
in this work that the large g- factor anisotropy gives rise to the gigantic
spin conversion anisotropy in Bi from experimental and theoretical approaches.
Spin-torque ferromagnetic resonance was applied to estimate the spin conversion
efficiency in rhombohedral (110) Bi to be 17%, which is unlike the negligibly
small efficiency in Bi(111). Harmonic Hall measurements supports the large spin
conversion efficiency in Bi(110). This is the first observation of gigantic
spin conversion anisotropy as the clear manifestation of the g-factor
anisotropy. Beyond the emblematic case of Bi, our study unveiled the
significance of the g-factor anisotropy in condensed-matter physics and can
pave a pathway toward establishing novel spin physics under g-factor control.
