To place electrodes easily and precisely to different subjects by adapting to individual differences of heads of the subjects to detect an electroencephalogram with accuracy.

The device 1 for ensuring attachment positions of a plurality of electrodes 5 attached to the head B of the subject A is equipped with a caplike body 2 of the jig placed on the head B of the subject A and a plurality of penetrating holes 3 which make the electrodes 5 pass through so that the electrodes 5 make contact with a scalp C at positions corresponding to the attachment position of each electrode 5 of the body 2.