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 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.