The invention includes a proxy server that constitutes means for providing, verifying and logging a digital signature for a message that is to be exchanged via a network, so that a security function for a digital signature can be implemented without changing an application program. In an example embodiment, a digital signature system comprises: applications for performing data processing; and a signature server connected to the applications via a LAN, wherein the signature server intercepts the message communication from the application to a destination device outside the LAN, provides a digital signature for a message document to be exchanged through communication, and transmits the obtained message document to the destination device.