1. An image scanner comprising: a scanning unit configured to scan images on document sheets; an image processing unit configured to perform image processing for the images scanned by the scanning unit under a predetermined condition; an identifying unit configured to identify one or more damaged sheets among the document sheets to be scanned; a condition setting unit configured to set a specific condition under which the image processing is to be performed for images scanned from the damaged sheets identified by the identifying unit; and a level acquiring unit configured to acquire a damage level that represents a degree to which the damaged sheets are damaged, wherein the condition setting unit is configured to set the specific condition depending on the damage level acquired by the level acquiring unit.