1. An observation method of observing luminescence or fluorescence emitted from a biological specimen, comprising: 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 and setting a single exposure time on the basis of divided times of the exposures, when the intensity is determined as being saturated in the intensity determining step; an image acquiring step of acquiring images same number of times as the number of divided exposures with the single exposure time set in the exposure dividing step; and an integrating step of integrating the images separately acquired in the image acquiring step into a single image having a number of bits in pixel values that is larger than a number of bits in pixel values of each of the images separately acquired.