An image input device for converting a light flux of an image observed with a microscope into a digital image, and for holding the digital image, comprising: an image capturing unit configured to: - form an image of the light flux output by a microscope unit, and - convert the formed image into digital data; a sensitivity changing unit configured to change a sensitivity of the image capturing unit; and a computing unit configured to: - obtain an exposure time by using the digital data captured by the image capturing unit in a state where the sensitivity is increased to a high sensitivity after instructing the sensitivity changing unit to increase the sensitivity of the image capturing unit to the high sensitivity while calculating the exposure time for obtaining a suitable brightness by the image capturing unit, and - issue an instruction to set the sensitivity of the image capturing unit to a low sensitivity after the exposure time is set in the image capturing unit, characterized in that: in the process of calculating the exposure time, the computing unit is further configured to: - calculate an intermediate exposure time on the basis of a ratio of brightness of the digital data, which is captured at the high sensitivity, to a suitable brightness, - repeat image capturing using the intermediate exposure time and a calculation of the intermediate exposure time until a brightness obtained on the basis of the digital data reaches the suitable brightness, and - obtain the exposure time on the basis of the intermediate exposure time if the brightness nearly matches the suitable brightness. an image capturing unit configured to: - form an image of the light flux output by a microscope unit, and - convert the formed image into digital data; - form an image of the light flux output by a microscope unit, and - convert the formed image into digital data; a sensitivity changing unit configured to change a sensitivity of the image capturing unit; and a computing unit configured to: - obtain an exposure time by using the digital data captured by the image capturing unit in a state where the sensitivity is increased to a high sensitivity after instructing the sensitivity changing unit to increase the sensitivity of the image capturing unit to the high sensitivity while calculating the exposure time for obtaining a suitable brightness by the image capturing unit, and - issue an instruction to set the sensitivity of the image capturing unit to a low sensitivity after the exposure time is set in the image capturing unit, characterized in that: - obtain an exposure time by using the digital data captured by the image capturing unit in a state where the sensitivity is increased to a high sensitivity after instructing the sensitivity changing unit to increase the sensitivity of the image capturing unit to the high sensitivity while calculating the exposure time for obtaining a suitable brightness by the image capturing unit, and - issue an instruction to set the sensitivity of the image capturing unit to a low sensitivity after the exposure time is set in the image capturing unit, characterized in that: in the process of calculating the exposure time, the computing unit is further configured to: - calculate an intermediate exposure time on the basis of a ratio of brightness of the digital data, which is captured at the high sensitivity, to a suitable brightness, - repeat image capturing using the intermediate exposure time and a calculation of the intermediate exposure time until a brightness obtained on the basis of the digital data reaches the suitable brightness, and - obtain the exposure time on the basis of the intermediate exposure time if the brightness nearly matches the suitable brightness. - calculate an intermediate exposure time on the basis of a ratio of brightness of the digital data, which is captured at the high sensitivity, to a suitable brightness, - repeat image capturing using the intermediate exposure time and a calculation of the intermediate exposure time until a brightness obtained on the basis of the digital data reaches the suitable brightness, and - obtain the exposure time on the basis of the intermediate exposure time if the brightness nearly matches the suitable brightness.