  Studies of gravitational microlensing are usually based on the lens equation,
which is valid only to first order in the gravitational constant $G$. However,
the amplification factor of microlensing is a second-order quantity with
respect to $G$. Despite this fact, conventional studies are still based on the
lowest-order lens equation. Then the question naturally arises: Why are these
conventional studies justified? We carefully study the relativistic correction
to the amplification factor at $O(G^2)$. We show that the amplification factor
for each image is corrected. However, the total amplification remains unchanged
at this order.
