An attaching device for holding electrodes for detecting EEG signals comprises a strap including a plurality of holes for accommodating electrodes and a rotary closure connecting a first end of the strap and a second end of the strap. The rotary closure includes a reference electrode and/or a ground electrode which are arranged centrally on a first surface of the rotary closure. The reference electrode and/or the ground electrode are in contact with a forehead when the attaching device is placed on a head and are positioned substantially centrally on a center line of the head.
