  We investigate the finite source size effect in the context of the wave
optics in the gravitational lensing. The magnification of an extended source is
presented in an analytic manner for the singular isothermal sphere lens model
as well as the point mass lens model with the use of the thin lens
approximation. The condition that the finite source size effect becomes
substantial is demonstrated. As an application, we discuss possible
observational consequences of the finite source size effect on astrophysical
systems.
