Abstract: The present invention concerns an alarm device for computer units (13a, 13b) which, via a network of twisted pair type, are connected to a common hub (3). The computer units (13a 13b) are arranged to be connected to a first alarm unit (4) corresponding to the respective computer unit. This incorporates an input (25) for connection to the hub, an output (23) for operational connection to a network board in a computer unit corresponding to the alarm unit, and a connection (22) for interconnection with a second alarm unit (5; 7) or a connection unit (10). In the device according to the invention, an alarm center (1) is operationally connected to the hub, which is arranged to transmit a request generated in the alarm center to the first alarm unit (4) and to notify the alarm center when a change occurs in the computer units (13a, 13b) associated with the alarm centers.