(Amended) A protective-control measuring system which comprises a plurality of devices including a protective-control measuring device that performs protective-control measurement on a power system, the plurality of devices being connected together over a transmission path and exchanging data one another, wherein each device comprises: a key-data storage unit which stores a set number of key data, the set number being predetermined; an authentication-tag generator unit which generates an authentication tag by using a transmission target main data and a piece of key data stored in the key-data storage unit; a transmitter/receiver unit which adds the generated authentication tag to the main data used for generating the authentication tag in order to generate transmission data, sends the transmission data to the transmission path, receives data from the transmission path, and divides the received data into the main data and the authentication tag; and a received-data authentication unit which authenticates a validity of the received data using the main data and the authentication tag both in the received data received by the transmitter/receiver unit, and the key data stored in the key-data storage unit, and the protective-control measuring system employs, as an information-theoretic security scheme, a scheme of changing the key data to be used every time upon generation of the authentication-tag by the authentication-tag generator unit, and also employs use-time restriction of prohibiting the use of each key data beyond an upper limit set time of reuse of the key data set in advance in a set period which is predetermined corresponding to an operating period of the protective-control measuring device, the set number of the key data is determined corresponding to a total sending time of transmission data during the set period and the upper limit set time of reuse of the key data, and a key data size is determined corresponding to the set number of the key data, and thus upon starting operation, the key-data storage unit stores the set number of the key data each having the key data size. a key-data storage unit which stores a set number of key data, the set number being predetermined; an authentication-tag generator unit which generates an authentication tag by using a transmission target main data and a piece of key data stored in the key-data storage unit; a transmitter/receiver unit which adds the generated authentication tag to the main data used for generating the authentication tag in order to generate transmission data, sends the transmission data to the transmission path, receives data from the transmission path, and divides the received data into the main data and the authentication tag; and a received-data authentication unit which authenticates a validity of the received data using the main data and the authentication tag both in the received data received by the transmitter/receiver unit, and the key data stored in the key-data storage unit, and the protective-control measuring system employs, as an information-theoretic security scheme, a scheme of changing the key data to be used every time upon generation of the authentication-tag by the authentication-tag generator unit, and also employs use-time restriction of prohibiting the use of each key data beyond an upper limit set time of reuse of the key data set in advance in a set period which is predetermined corresponding to an operating period of the protective-control measuring device, the set number of the key data is determined corresponding to a total sending time of transmission data during the set period and the upper limit set time of reuse of the key data, and a key data size is determined corresponding to the set number of the key data, and thus upon starting operation, the key-data storage unit stores the set number of the key data each having the key data size.