This invention shortens a time required for batch verification of a plurality of digital documents. To this end, whether or not an electronic signature in a long-term signature format is added to a digital document is checked. A first verification process is applied to a digital document which is determined to have an electronic signature in the long-term signature format added to it. A second verification process, which requires a shorter processing time than the first verification process, is applied to a digital document which is determined not to have an electronic signature in the long-term signature format added to it.