An image scanner includes a scanning unit that scans a white scanning member via a contact glass on one line basis when the image scanner is powered on and outputs image data of the white scanning member, and a determining unit that determines whether at least one of the contact glass and the white scanning member has dirt or a scratch thereon by comparing the image data with reference data on one line basis. The reference data is acquired by scanning the scanning member in a situation that there is no dirt or scratch on the scanning member.