A security device (20; 120) for supporting secure communication via a field bus, comprising: a connection means (10; 121) for directly coupling the security device (20, 120) to a network interface (31; 81) of a fieldbus subscriber (30; 80) the network interface (31; 81) being adapted to be connected to a fieldbus; and the field bus subscriber (30; 80) being not adapted for secure communication via the fieldbus, a network interface (50; 122) for connecting the security device (20, 120) to the field bus; a transmitting and receiving device (24; 123) configured to securely transfer data coming from the directly coupled fieldbus subscriber that is not adapted for secure communication, via the fieldbus in accordance with a predetermined security protocol, and furthermore configured to receive data transferred via the fieldbus in accordance with the predetermined security protocol and intended for the fieldbus subscriber and to deliver them to the fieldbus subscriber; characterized in that, in the coupled state, a link is established between the security device (20; 120) and the field bus subscriber such that if the coupling is disconnected or damaged, proper operation of the security device (20; 120) is blocked reversibly or irreversibly; and that the security device (20; 120) further comprises a control device (22; 126) and a monitoring device (21; 124) configured for monitoring the link between the security device (20; 120) and a connected field bus subscriber, wherein the control device (22; 126) is configured to block proper operation of the security device in response to an error signal from the monitoring device (21; 124). a connection means (10; 121) for directly coupling the security device (20, 120) to a network interface (31; 81) of a fieldbus subscriber (30; 80) the network interface (31; 81) being adapted to be connected to a fieldbus; and the field bus subscriber (30; 80) being not adapted for secure communication via the fieldbus, a network interface (50; 122) for connecting the security device (20, 120) to the field bus; a transmitting and receiving device (24; 123) configured to securely transfer data coming from the directly coupled fieldbus subscriber that is not adapted for secure communication, via the fieldbus in accordance with a predetermined security protocol, and furthermore configured to receive data transferred via the fieldbus in accordance with the predetermined security protocol and intended for the fieldbus subscriber and to deliver them to the fieldbus subscriber; characterized in that, in the coupled state, a link is established between the security device (20; 120) and the field bus subscriber such that if the coupling is disconnected or damaged, proper operation of the security device (20; 120) is blocked reversibly or irreversibly; and that the security device (20; 120) further comprises a control device (22; 126) and a monitoring device (21; 124) configured for monitoring the link between the security device (20; 120) and a connected field bus subscriber, wherein the control device (22; 126) is configured to block proper operation of the security device in response to an error signal from the monitoring device (21; 124).