An image scanning device for scanning an image on a document comprises a document table on which a document to be scanned is placed, a scanning unit which is installed in the document table for scanning an image on a document by projecting light from a light source onto the document in a time-sharing manner and detecting reflected light from the document, a moving unit which moves at least one of the document and the scanning unit to let them move relatively with each other, and a document type judgment unit which judges the type of the document in regard to transmission of light through the document based on a non-lighting period image signal which is outputted by the scanning unit by scanning the document for one line in a main scanning direction in a non-lighting period of the light source.