When a biological specimen, such as living cells, biological tissue, or a small animal, is quantitatively observed, the specimen is observed for a long time or over a long period, while maintaining the quantitative properties. Provided is an observation method of observing luminescence or fluorescence emitted from a biological specimen. The observation method includes a referring step of referring to image data previously acquired; an intensity determining step of determining whether the intensity of the image data previously acquired is saturated; an exposure dividing step of dividing the exposure of the image data previously acquired one time into a plurality of exposures when the intensity is determined as being saturated in the intensity determining step; an image acquiring step of acquiring images the same number of times as the number of divided exposures; and an integrating step of integrating the images separately acquired multiple times into a single image.