Abstract: A security device to support secure communication via a field bus, has a connecting apparatus for the direct coupling of the security device to a network interface of a field bus subscriber, which is formed for connecting to a field bus and which is not formed for secure communication via the field bus. In the coupled state, there is a link between the security device and the field bus subscriber such that, if the link is disconnected or damaged, proper operation of the security device is reversibly or irreversibly blocked. Further, a transmitting and receiving apparatus is provided which is formed to securely transfer data coming from a directly coupled field bus participant, which is not formed for secure communication, via the field bus according to a predetermined security protocol, and which is further formed to receive data transferred via the field bus and intended for the field bus participant according to the predetermined security protocol and to deliver them to the field bus participant.