  We study gravitational lensing of gravitational waves taking into account the
spin of a graviton coupled with a dragged spacetime made by a rotating object.
We decompose the phase of gravitational waves into helicity-dependent and
independent components with spin optics, analyzing waves whose wavelengths are
shorter than the curvature radius of a lens object. We analytically confirm
that the trajectory of gravitational waves splits depending on the helicity,
generating additional time delay and elliptical polarization onto the
helicity-independent part. We exemplify monotonic gravitational waves lensed by
a Kerr black hole and derive the analytical expressions of corrections in phase
and magnification. The corrections are enhanced for longer wavelengths,
potentially providing a novel probe of rotational properties of lens objects in
low-frequency gravitational-wave observations in the future.
