An apparatus includes a memory; and a processor coupled to the memory and configured to generate a first common key whose key value varies based on a first elapsed time when a notification of the first elapsed time after a start-up of another apparatus to which a data frame to be encrypted is to be transmitted has been made, generate a second common key whose key value varies based on a second elapsed time after a start-up of the apparatus when a notification of the first elapsed time has not been made, and encrypt the data frame by any one of the first common key and the second common key as a common key and transmit the encrypted data frame to the another apparatus.