The image processing apparatus comprises a document reading portion that optically reads an original document and inputs image data, a digital signature adding control portion that adds a digital signature to the read image data, a reading condition value setting portion that sets reading condition values by the kind of original document at the time of reading the original document by the document reading portion, and a reading condition matching determining portion that determines whether the set reading condition values match predetermined reading condition standard values. The digital signature adding control portion does not add the digital signature to the image data read at the document reading portion when, as a result of the determination by the reading condition matching determining portion, the reading condition values do not match the predetermined reading condition standard values.