An ultra-miniature in-vivo electrode used for measuring the bioelectrical neural activity etc. in a living body with small cubic volume, slight burden to the living body, and high insulation performance, thereby facilitating long term stable measurements. Structurally the in-vivo electrode comprises conductors 13, 14 made of polymeric material having a through-hole 19 in each center so as to lead a measuring object 18; a intervening member 15 made of non-conductive polymeric material provided in between the conductors so as to insulate the periphery of the conductors; edge members 16, 17 provided in the outer edge of the conductors; a clad body 12 made of cylindrical insulant enclosing the periphery of them; measuring wires 21,22 to lead electricity to outside from each conductor 21, 22; a slit 26 which reaches the through-hole 19 from the outside surface being formed in all of the conductors, the intervening member, the edge members, and the clad body.