A safety system comprising: a process execution unit that executes safety control according to a safety program (SPG); a communication unit that is directly connected to the process execution unit and mediates external access to the safety program (SPG) stored in the process execution unit ; and a support device that is connected to the communication unit via a network and executes maintenance including an addition or a change with respect to the safety program (SPG) in accordance with a user operation, wherein the support device and the communication unit identify each other by using information (SK, PK, CR) exchanged in advance, and exchange data required for the maintenance, the communication unit stores a key (SK) as at least part of the information (SK, PK, CR) exchanged in advance, the communication unit stores a first secret key (SK1) as the key, and the support device stores at least one of a first public key (PK1) forming a pair with the first secret key (SK1), the support device generates a third key (TCK) and encrypts the safety program (SPG) as data required for the maintenance with the third key (TCK), encrypts the third key (TCK) to be decryptable with the key by the communication unit , and transmits the data and the third key (TCK) to the communication unit , the safety system further characterized in that the communication unit encrypts a second hash value calculated from the safety program (SPG) that has been received from the support device with the third key obtained from the support device that is decrypted with the key, and the support device decrypts and collates the encrypted second hash value (HS) received from the communication unit . a process execution unit that executes safety control according to a safety program (SPG); a communication unit that is directly connected to the process execution unit and mediates external access to the safety program (SPG) stored in the process execution unit ; and a support device that is connected to the communication unit via a network and executes maintenance including an addition or a change with respect to the safety program (SPG) in accordance with a user operation, wherein the support device and the communication unit identify each other by using information (SK, PK, CR) exchanged in advance, and exchange data required for the maintenance, the communication unit stores a key (SK) as at least part of the information (SK, PK, CR) exchanged in advance, the communication unit stores a first secret key (SK1) as the key, and the support device stores at least one of a first public key (PK1) forming a pair with the first secret key (SK1), the support device generates a third key (TCK) and encrypts the safety program (SPG) as data required for the maintenance with the third key (TCK), encrypts the third key (TCK) to be decryptable with the key by the communication unit , and transmits the data and the third key (TCK) to the communication unit , the safety system further characterized in that the communication unit encrypts a second hash value calculated from the safety program (SPG) that has been received from the support device with the third key obtained from the support device that is decrypted with the key, and the support device decrypts and collates the encrypted second hash value (HS) received from the communication unit . <EOS>