  We investigate the extended source size effects on gravitational lensing in
which a lens consists of a smooth potential and small mass clumps
(``substructure lensing''). We first consider a lens model that consists of a
clump modeled as a singular isothermal sphere (SIS) and a primary lens modeled
as an external background shear and convergence. For this simple model, we
derive analytic formulae for (de)magnification of circularly symmetric top-hat
sources with three types of parity for their lensed images, namely, positive,
negative, and doubly negative parities. Provided that the source size is
sufficiently larger than the Einstein radius of the SIS, we find that in the
positive (doubly negative) parity case, an extended source is always magnified
(demagnified) in comparison with the unperturbed macrolens system, whereas in
the negative parity case, the (de)magnification effect, which depends on the
sign of convergence minus unity is weaker than those in other parities. It is
shown that a measurement of the distortion pattern in a multiply lensed image
enables us to break the degeneracy between the lensing effects of clump mass
and those of clump distance if lensing parameters of the relevant macrolens
model are determined from the position and flux of multiple images. We also
show that an actual density profile of a clump can be directly measured by
analyzing the ``fine structure'' in a multiply lensed image within the Einstein
radius of the clump.
