1. An imaging device including an image sensor, comprising: an image obtaining control unit that obtains plural continuous images of an object taken by the image sensor; a size determining unit that determines a reduction ratio for reduced images with respect to the images taken by the image sensor based on photographing information of at least one of the images taken by the image sensor, and generates the reduced images by reduction of the images taken by the image sensor and obtained by the image obtaining control unit based on the determined reduction ratio; a motion vector detection unit that detects a motion vector of the reduced images; and an image synthesis unit that synthesizes the images obtained by the image obtaining control unit based on the motion vector detected by the motion vector detection unit to obtain a synthesized image.