Abstract: A concentrator for a local area network with loop topology uses a connector to which a first workstation is connected and a relay contact for switching between a state in which second and third workstations positioned at both sides of the first workstation are connected to each other and another state in which the first workstation is connected via the connector between the second and the third workstations. The concentrator includes an activation member for activating the relay contact and a pair of condensers to be charged by direct current from the first workstation when the first workstation is connected. The activation member is operated by discharging the direct current. A controller controls the discharging of one of the pair of condensers by detecting the changes in connection conditions of the first workstation.