1. An imaging device comprising: a movable stage capable of moving in two different directions while mounting an object to be inspected; a camera which is arranged in face-to-face relation to the movable stage, photographs the object to be inspected, and generates a moving picture image made of a plurality of sequential frame images; a mosaic image generating processing device which assembles not less than two still images photographed by the camera, and generates a mosaic image that is wider in field of view than the actual field of view of the camera; a relative position determination unit which determines a relative position between the frame image and the mosaic image; a display reduction unit comprising a processor configured to average pixel values and to reduce the frame image and the mosaic image respectively based on the image size of the mosaic image, and generates a display frame image and a display mosaic image; a live image display unit which updates a display position of the display frame image with respect to the display mosaic image based on a determination result of the relative position, and displays a moving picture image configured by the display frame images on the display mosaic image, a still image obtaining unit which obtains a still image photographed by the camera based on a fetch instruction while displaying the moving picture image on the display mosaic image by the live image display unit; and wherein the mosaic image generating processing device determines a relative position between the still image and the mosaic image, and assembles the still image and the mosaic image to generate a new mosaic image, wherein a reduction ratio is set so that the whole of the display mosaic image and the whole of the display frame image, at any position that has an overlap area necessary for determination of the relative position, are displayed at the same time.