First, image capturing is performed in the exposure time of ExpStart. If the maximum intensity value of all the pixels of an image is smaller than TransStart, the exposure time is increased and the image capturing is performed again. After that, increase in the exposure time and the image capturing are repeatedly performed until the maximum intensity value of all the pixels of the captured optical image becomes equal to or greater than TransStart. When the maximum intensity value becomes equal to or greater than TransStart, image data is transferred to a computer to be recorded on the hard disk, and the number of images recorded is counted. Subsequently, the increase in the exposure time, the image capturing, transfer and record of the image data, and count of the number of images recorded are repeatedly performed. The image capturing is finished when the minimum intensity value in an area of the fluorescent image that is smaller than the entire area becomes larger than TransEnd, the number of images recorded becomes larger than TransPicts, or the exposure time becomes longer than MaxExpTime.