A data backup system includes a mobile device, a data storing apparatus for backup-saving user data used in the mobile device, and a user management apparatus for managing user information about a user using the mobile device, where the device and apparatuses communicate with each other. When the mobile device is a replacement for an old mobile device, the user management apparatus stores and manages a device ID of the mobile device in association with an old device ID of the old mobile device and personal identification information of a user using the mobile device and who has used the old mobile device. When downloading old user data previously used in the old mobile device from the data storing apparatus, the mobile device creates and transmits a user data acquiring request message with an old device user data acquiring request flag activated to the data storing apparatus. If the flag is set in the user data acquiring request message, the data storing apparatus verifies identification of the user with respect to the user management apparatus. If the identification of the user of the mobile device and the old mobile device is verified, the data storing apparatus retrieves and transmits the old user data to the mobile device.