A first node apparatus in a network constituted by a plurality of node apparatuses including the first node apparatus and a second node apparatus, the first node apparatus comprising: an access key generation unit configured to change and generate a first access key being a cryptographic key unique to the first node apparatus at intervals of a first time; a shared key generation unit configured to change and generate a shared key being same for the plurality of node apparatuses in the network at intervals of a second time being same for the plurality of node apparatuses; an access key notification unit configured to encrypt, using the generated shared key and transmit, to the second node apparatus, the generated first access key; an access key reception unit configured to receive an access key notification frame that is transmitted from the second node apparatus and that includes access key notification data being data obtained by encrypting a second access key being a cryptographic key unique to the second node apparatus using the shared key; an access key decryption unit configured to obtain the second access key from the access key notification data by decrypting the access key notification data using the generated shared key; a data transmission unit configured to attach, to a first plaintext frame, first signature data obtained by encrypting, using the shared key, data including a first hash value calculated from the first plaintext frame, and to encrypt the first plaintext frame, to which the first signature data is attached, into a first encrypted frame using the second access key obtained by decryption, and to transmit the first encrypted frame; a data reception unit configured to receive, from the second node apparatus, a second encrypted frame obtained by encrypting, by the first access key, a second plaintext frame to which a second signature data obtained by encrypting data including a second hash value by the shared key is attached; a data decryption unit configured to decrypt the second encrypted frame by the first access key to obtain the second plaintext frame, to which the second signature data is attached, from the second encrypted frame; and a consistency confirmation unit configured to obtain the second hash value by decrypting the second signature data using the generated shared key, to calculate a third hash value from the second plaintext frame, and to confirm whether or not the second hash value and the third hash value are consistent with each other. an access key generation unit configured to change and generate a first access key being a cryptographic key unique to the first node apparatus at intervals of a first time; a shared key generation unit configured to change and generate a shared key being same for the plurality of node apparatuses in the network at intervals of a second time being same for the plurality of node apparatuses; an access key notification unit configured to encrypt, using the generated shared key and transmit, to the second node apparatus, the generated first access key; an access key reception unit configured to receive an access key notification frame that is transmitted from the second node apparatus and that includes access key notification data being data obtained by encrypting a second access key being a cryptographic key unique to the second node apparatus using the shared key; an access key decryption unit configured to obtain the second access key from the access key notification data by decrypting the access key notification data using the generated shared key; a data transmission unit configured to attach, to a first plaintext frame, first signature data obtained by encrypting, using the shared key, data including a first hash value calculated from the first plaintext frame, and to encrypt the first plaintext frame, to which the first signature data is attached, into a first encrypted frame using the second access key obtained by decryption, and to transmit the first encrypted frame; a data reception unit configured to receive, from the second node apparatus, a second encrypted frame obtained by encrypting, by the first access key, a second plaintext frame to which a second signature data obtained by encrypting data including a second hash value by the shared key is attached; a data decryption unit configured to decrypt the second encrypted frame by the first access key to obtain the second plaintext frame, to which the second signature data is attached, from the second encrypted frame; and a consistency confirmation unit configured to obtain the second hash value by decrypting the second signature data using the generated shared key, to calculate a third hash value from the second plaintext frame, and to confirm whether or not the second hash value and the third hash value are consistent with each other.