A data backup system comprising: a mobile device (21, 22); a data storing apparatus configured to store user backup data uploaded from the mobile device; and a user management apparatus configured to manage user information of a user of the mobile device in association with mobile device ID information, the mobile device, the data storing apparatus, and the user management apparatus being capable of mutually communicating with each other; wherein the mobile device is configured to transmit a user ID information re-registration message to the user management apparatus when the mobile device is a replacement for a previously used old mobile device, the user ID information re-registration message including new device ID information of the mobile device, the user information of the user, and old device ID information of the old mobile device; wherein the user management apparatus is further configured, upon receiving the user ID information re-registration message from the mobile device, to store and manage the new device ID information of the mobile device, in association with the user information of the user and the old device ID information, and to create a user device list representing that the user of the new mobile device and the user of the old mobile device are identical, wherein the mobile device is configured to create and transmit a user data acquiring request message with an old device user data acquiring request flag activated to the data storing apparatus when downloading user data previously used in the old mobile device, the user data acquiring request message including the new ID information of the mobile device; and wherein the data storing apparatus is configured to transmit a device ID verification request message containing the new device ID information taken out of the data acquiring request message to the user management apparatus to verify identification of the user when the flag is set in the user data acquiring request message; the user management apparatus is configured to retrieve the user data having the same ID information as the new ID information contained in the data acquiring request message in the user device list, and, if retrieved, transmit a device ID verification response message containing the old mobile ID information to the data storing apparatus to verify the identification of the user; and the data storing apparatus is further configured to retrieve the user data used in the old mobile device based on the old device ID information contained in the device ID verification response message, and transmit the retrieved user data to the mobile device. a mobile device (21, 22); a data storing apparatus configured to store user backup data uploaded from the mobile device; and a user management apparatus configured to manage user information of a user of the mobile device in association with mobile device ID information, the mobile device, the data storing apparatus, and the user management apparatus being capable of mutually communicating with each other; wherein the mobile device is configured to transmit a user ID information re-registration message to the user management apparatus when the mobile device is a replacement for a previously used old mobile device, the user ID information re-registration message including new device ID information of the mobile device, the user information of the user, and old device ID information of the old mobile device; wherein the user management apparatus is further configured, upon receiving the user ID information re-registration message from the mobile device, to store and manage the new device ID information of the mobile device, in association with the user information of the user and the old device ID information, and to create a user device list representing that the user of the new mobile device and the user of the old mobile device are identical, wherein the mobile device is configured to create and transmit a user data acquiring request message with an old device user data acquiring request flag activated to the data storing apparatus when downloading user data previously used in the old mobile device, the user data acquiring request message including the new ID information of the mobile device; and wherein the data storing apparatus is configured to transmit a device ID verification request message containing the new device ID information taken out of the data acquiring request message to the user management apparatus to verify identification of the user when the flag is set in the user data acquiring request message; the user management apparatus is configured to retrieve the user data having the same ID information as the new ID information contained in the data acquiring request message in the user device list, and, if retrieved, transmit a device ID verification response message containing the old mobile ID information to the data storing apparatus to verify the identification of the user; and the data storing apparatus is further configured to retrieve the user data used in the old mobile device based on the old device ID information contained in the device ID verification response message, and transmit the retrieved user data to the mobile device.