A time stamping system for electronic documents has a document preparation device and a time stamp issuing server. The document preparation device includes an electronic document reading device that reads an electronic document. A digest value computer computes a digest value based on the read document. A transmitter transmits the computed digest value to the time stamp server with a request for a time stamp. A receiver receives the requested time stamp from the time stamp server and an electronic writer writes the received time stamp into the electronic document.