An image scanning apparatus, comprising: an image scanning unit that is configured to optically scan a document image with a specified resolution and generates image data of the document image; a display device that is configured to display an operation screen to a user; an input device that is configured to receive a user operation; a visibility adjusting unit that is configured to (a) display on the display device a user interface to specify a visibility level, (b) detect with the input device a user operation performed to the user interface, the user operating specifying a visibility level, (c) determine the visibility level based on the detected user operation, and (d) convert the determined visibility level to a resolution in accordance with a predetermined conversion rule; and a scan control unit that is configured to specify to the image scanning unit the resolution obtained in the conversion performed by the visibility adjusting unit, to cause the image scanning unit to generate image data of the document image, and to convert the generated image data to image data of a specified data type; and a non volatile storage device, characterized in that the conversion rule is set for each of image types that include character, photograph, and combination of character and photograph; the visibility adjusting unit is configured to detect with the input device a user operation to specify an image type of the document image, to determine the image type corresponding to the detected user operation, and to convert the determined visibility level to the resolution in accordance with the conversion rule set for the determined image type; the conversion rule indicates a relationship between visibility levels and resolutions, and a plurality of conversion rules are set in conversion rule data stored in the non volatile storage device correspondingly to combinations of color mode, data format, and the image type, respectively. an image scanning unit that is configured to optically scan a document image with a specified resolution and generates image data of the document image; a display device that is configured to display an operation screen to a user; an input device that is configured to receive a user operation; a visibility adjusting unit that is configured to (a) display on the display device a user interface to specify a visibility level, (b) detect with the input device a user operation performed to the user interface, the user operating specifying a visibility level, (c) determine the visibility level based on the detected user operation, and (d) convert the determined visibility level to a resolution in accordance with a predetermined conversion rule; and a scan control unit that is configured to specify to the image scanning unit the resolution obtained in the conversion performed by the visibility adjusting unit, to cause the image scanning unit to generate image data of the document image, and to convert the generated image data to image data of a specified data type; and a non volatile storage device, characterized in that the conversion rule is set for each of image types that include character, photograph, and combination of character and photograph; the visibility adjusting unit is configured to detect with the input device a user operation to specify an image type of the document image, to determine the image type corresponding to the detected user operation, and to convert the determined visibility level to the resolution in accordance with the conversion rule set for the determined image type; the conversion rule indicates a relationship between visibility levels and resolutions, and a plurality of conversion rules are set in conversion rule data stored in the non volatile storage device correspondingly to combinations of color mode, data format, and the image type, respectively.