In the present invention, cells cultured inside the wells of a microplate are observed easily and clearly regardless of the height or curvature state of the liquid surface of the culturing liquid. Provided is an observation device that includes an illumination optical system that irradiates a transparent sample with illumination light from a light source, an objective lens that collects observation light from the sample, a detection optical system that detects the observation light collected by the objective lens, and a retroreflective member that is arranged opposite the objective lens with the sample interposed therebetween and in which a plurality of very small reflective elements are arrayed. The objective lens and the illumination optical system are arranged below the sample in the direction of gravity.