1. An imaging device comprising: an image pick-up device for imaging a chemiluminescent imaging subject; a pre-exposure section that performs pre-exposure of the image pick-up device to the imaging subject a plurality of times to acquire a plurality of pre-exposure images; a light emission characteristic calculation section that calculates the light emission characteristics of the chemiluminescent imaging subject that attenuate with the passage of time based on the plurality of pre-exposure images; an exposure time calculation section that calculates an exposure time for main exposure corresponding to a target density for the imaging subject based on the light emission characteristics; and a main exposure section that performing main exposure to the imaging subject with the calculated exposure time so as to acquire a main exposure image, wherein the pre-exposure is re-executed at least with the pixel summing number for pre-exposure increased or with the exposure time for pre-exposure increased when there is no change between the plurality of pre-exposure images, and wherein, if the calculated exposure time is not lower than a predetermined value, the exposure time calculation section reduces the target density of the imaging subject and calculates the exposure time for main exposure corresponding to the target density for the imaging subject again.