1. An image processing apparatus that aligns a plurality of images, which are obtained by continuously shooting a subject, and then synthesizes the plurality of images to obtain a composite image, comprising: an image decimation unit configured to sample the plurality of images at predetermined time intervals and select at least two images among the plurality of images as decimation images; a moving vector detection unit configured to detect a moving vector, which is indicative of a moving direction and a moving amount of the subject, between the decimation images; a decimation control unit configured to change decimation conditions, on which said image decimation unit performs the sampling, according to an absolute value of the moving vector which is an output from said moving vector detection unit; and a synthesizing unit configured to align the plurality of images based on the output from said moving vector detection unit and then synthesize the aligned plurality of images to obtain the composite image.