A time stamp obtaining apparatus for maintaining the certificate of the existence of electronic filing document including: a receiving unit (12) receiving hash value of the document data; a transmitting section (15c) transmitting the hash value to time stamp providing apparatus (30a); a storing processing section (15d) inserting the time stamp encoded with private key in unupdated data (16b) concerning time stamp received since point to which private key is updated and storing; an update detecting section (11) detecting the update of private key; a calculation section (13b) calculating hash value for all time stamps included in the unupdated data (16b); a transmitting section (13c) transmitting the hash value; a storing processing section (13d) moving the unupdated data (16b) to past data(16c) concerning time stamp received before point to which private key is updated last time when time stamp is received, storing it, and storing the time stamp newly received as new unupdated data (16b); is provided.