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.