An observation device includes: an illumination unit that generates illumination light to be radiated onto cells floating in a culture fluid inside a culture vessel; a light receiving unit that receives observation light coming from the cells that have been irradiated with the illumination light, the observation light being imaged by an image-forming optical system; and a casing that has a transmissive section so as to transmit the illumination light and the observation light and that accommodates the illumination unit and the light receiving unit, wherein the casing has an elongated cylindrical form that is configured to be inserted into the culture fluid via a port used to insert a tube into the culture vessel.