1. An image synthesizing apparatus, comprising: an image acquisition unit that acquires a plurality of images being temporally continuous; a selection unit that selects a single image serving as reference of brightness, from the plurality of images acquired by the image acquisition unit; a detection unit that compares the single image selected by the selection unit with other images acquired by the image acquisition unit, and detects an image with a difference value being higher than a predetermined threshold value as a result of such comparison; a setting unit that sets a synthetic rate, based on the image detected by the detection unit; and an image generation unit that generates an image by executing synthesis of the plurality of images acquired by the image acquisition unit including the image detected by the detection unit, by using the synthetic rate that is set by the setting unit.