A white reference image is inputted to an image sensor so as to output groups of analog data. The groups of analog data are converted by a processing unit into those groups of digital data the number of which is smaller than that of photosensor elements of the image sensor. The lowest density value of the groups or digital data is inspected, an operating environment is set so that the lowest density value of the groups of digital data may become a density value within a predetermined range. An optical system, a drive unit and the processing unit are controlled under the operating environment so as to output from the processing unit, image data which express a subject image inputted to the image sensor.