In a sensor network system, a transmitter device uses a message authentication key generated by a message generator to transmit a message with authenticator to plural receiver devices, which in reply produce a certification by a certification generator from a message with authenticator held by a message holder to transmit the certification to the transmitter device. An information generator of the transmitter device uses an identification from a reception checker and the message authentication key thus generated to generate an authentication key notification, which will be transmitted to receiver devices having transmitted the certification. In the receiver devices, the message holder holds the authentication key notification, from which an authentication key acquirer acquires the message authentication key, which a message authenticator uses to compare the message with authenticator generated with the message with authenticator held in the message holder to confirm the received message.