Abstract: A data management method includes receiving, by a terminal device, data stored in a first storage provided in an information processing device and storing the received data in a second storage provided in the terminal device, transmitting, to the information processing device, differential data between the data stored in the second storage and first updated data obtained by updating the data stored in the second storage, creating first encrypted data by encrypting the first updated data, deleting part of the created first encrypted data from the second storage, creating, by the information processing device, second encrypted data by encrypting second updated data obtained by updating the differential data transmitted from the terminal device, transmitting, to the terminal device, part of the second encrypted data which is identical in contents to the part of the first encrypted data deleted by the terminal device.