  In the gravitational lensing of gravitational waves, the wave optics should
be used instead of the geometrical optics when the wavelength $\lambda$ of the
gravitational waves is longer than the Schwarzschild radius of the lens mass
$M_L$. For the gravitational lensing of the chirp signals from the coalescence
of the super massive black holes at the redshift $z_S\sim 1$ relevant to LISA,
the wave effects become important for the lens mass smaller than $\sim 10^8
M_{\odot}$. For such cases, we compute how accurately we can extract the mass
of the lens and the source position from the lensed signal. We consider two
simple lens models: the point mass lens and the SIS (Singular Isothermal
Sphere). We find that the lens mass and the source position can be determined
within $\sim 0.1% [(S/N)/10^3]^{-1}$ for the lens mass larger than $10^8
M_{\odot}$ and $\gsim 10% [(S/N)/10^3]^{-1}$ for the lens mass smaller than
$10^7 M_{\odot}$ due to the diffraction effect, where $(S/N)$ is the signal to
noise ratio of the unlensed chirp signals. For the SIS model, if the source
position is outside the Einstein radius, only a single image exists in the
geometrical optics approximation so that the lens parameters can not be
determined. While in the wave optics cases we find that the lens mass can be
determined even for $M_L < 10^8 M_{\odot}$. For the point mass lens, one can
extract the lens parameters even if the source position is far outside the
Einstein radius. As a result, the lensing cross section is an order of
magnitude larger than that for the usual strong lensing of light.
