  Gravitational waves propagate along null geodesics like light rays in the
geometrical optics approximation, and they may have a chance to suffer from
gravitational lensing by intervening objects, as is the case for
electromagnetic waves. Long wavelength of gravitational waves and compactness
of possible sources may enable us to extract information in the interference
among the lensed images. We point out that the interference term contains
information of relative transverse velocity of the source-lens-observer system,
which may be obtained by possible future space-borne gravitational wave
detectors such as BBO/DECIGO.
