1. An image synthesizing apparatus, comprising: an image acquisition unit that acquires a plurality of images that are temporally continuous; a selection unit that selects a reference image from the plurality of images acquired by the image acquisition unit; a detection unit that compares the reference image selected by the selection unit with other images acquired by the image acquisition unit, and detects an image having an image area with a difference value that is higher than a predetermined threshold value as a result of the comparison; a setting unit that sets a transparence intensity of the image area in the image detected by the detection unit; and an image generation unit that generates an image by executing additive synthesis of the plurality of images acquired by the image acquisition unit including the image detected by the detection unit by using the transparence intensity that is set by the setting unit for the image area in the image detected by the detection unit.