  The extended source effect on microlensing magnification is non-negligible
and must be taken into account for in an analysis of microlensing. However, the
evaluation of the extended source magnification is numerically expensive
because it includes the two-dimensional integral over source profile. Various
studies have developed methods to reduce this integral down to the
one-dimensional-integral or integral-free form, which adopt some approximations
or depend on the exact form of the source profile, e.g. disk, linear/quadratic
limb-darkening profile. In this paper, we develop a new method to evaluate the
extended source magnification based on fast Fourier transformation (FFT), which
does not adopt any approximations and is applicable to any source profiles. Our
implementation of the FFT based method enables the fast evaluation of the
extended source magnification as fast as $\sim1$ msec (CPU time on a laptop)
and guarantees an accuracy better than 0.3%. The FFT based method can be used
for the template fitting to a huge data set of light curves from the existing
and upcoming surveys.
