To improve the reliability of print relating to an electronic signature by calculating a value intrinsic to file data by using a unidirectional function from the data of a file with a signature electronically signed by a secret key, adding the value intrinsic to the file data to print data and generating intrinsic print data.

At the time of printing received electronic mail, whether normal print 901 is selected or electronic signature print 902 is selected is judged at the time of the operation of printing execution 900. Then, in the case that the electronic signature print 902 is selected, the message digest value of received electronic mail data is calculated and added to the print data by a prescribed form. Then, the print data are generated by adding the calculated message digest value by low density characters so as to be difficult to be visually recognized in the entire print image area and printing is finally executed.