An image-reading device includes a document-supporting base, a document-pressing member, a reading unit, a controlling unit, and a determining unit. The document-supporting base has a transparent member on which a document is placed. The document-pressing member is disposed on the document-supporting base so as to cover the transparent member. The document-pressing member is capable of opening and closing on the document-supporting base. The reading unit has a light source and capable of reading an image from the document placed on the transparent member while the light source irradiates light on the document. The controlling unit is configured to control the reading unit to execute a first scanning operation to output image signals for the image on the document while the light source is lit, and a second scanning operation to output image signals for the image on the document while the light source is not lit. The determining unit is configured to determine whether outside light has penetrated the document-supporting base based on the image signals for the second scanning operation outputted from the reading unit.