A phase difference observation apparatus includes a light source; an illumination guide to guide illumination light from the light source to an observation target object in a cell culture vessel; an optical imager to form an optical image of the observation target object on an image sensor; and a controller. The illumination guide includes a spatial modulator to change an intensity distribution of the illumination light; the controller contains intensity distribution correction information associating a position of the imaging guide with respect to the cell culture vessel with an intensity distribution of illumination light at the position of the optical imager; the controller acquires imaging system position information, which is the position of the optical imager; and the controller changes an intensity distribution of illumination light in the spatial modulator on the basis of the imaging system position information and the intensity distribution correction information.