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

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... We show that the amplification factor for each image is corrected. However, the total amplification remains unchanged at this order.