an observation device that observes a test specimen such as cultured cells or the like includes : an illumination device that illuminates the test specimen ; an image - capturing device that acquires an image of the test specimen illuminated by the illumination device ; a storage unit that stores correlation data manifesting a correlation relationship between an occupation ratio of the test specimen within a culture vessel , and statistically processed data regarding to luminance information of the test specimen ; and a calculation unit that obtains the statistically processed data based upon luminance information in a specified color of the test specimen captured by the image - capturing device , and calculates the occupation ratio of the test specimen within the culture vessel using the correlation data in the storage unit .