Image observation device and microscope system

An image observation device configured to:

• perform a reference-image generating that generates a reference image on a basis of one or more two-dimensional images of a Z-stack image constituted of the plurality of two-dimensional images that are acquired by shifting a focal position in an optical-axis direction of an objective lens;
• perform a frame generating that generates, on the generated reference image, a frame that has a plurality of window portions covering different small regions of the reference image and that is superimposed on the reference image;
• perform a frame operating that allows a user to operate the generated frame so as to be moved with respect to the reference image;
• perform a focal-range allocating that allocates different focal ranges to the respective window portions of the generated frame;
• perform an omnifocal-image generating that generates, for the respective small regions covered by the respective window portions, omnifocal images by using, among the two-dimensional images in regions corresponding to the respective small regions, the two-dimensional images in the focal ranges allocated to the respective window portions;
• perform an image replacing that replaces the regions of the reference image in the respective window portions with the respective generated omnifocal images that are based on the different focal ranges;
• and
• display the reference image, the frame, and the omnifocal images.