The present invention provides an imaging device capable of displaying the whole of a display mosaic image in a screen even when the image size of the mosaic image is largely changed, the imaging device including: a mosaic image generation unit which assembles a plurality of still images, and generates the mosaic image; a relative position determination unit which determines a relative position between a frame image and the mosaic image; a display reduction unit which reduces 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; and a live image display unit which updates a display position of the frame image with respect to the mosaic image based on a determination result of the relative position.