A multipolar single-head plug having a bar electrode exposed at a tip thereof, tubular electrodes concentrically arranged on an outer circumference of the bar electrode and exposed at the outer circumference on a side closer to a root than the exposed part of the bar electrode at the tip and an insulator composed of a first insulating part, and second insulating parts. A middle section of at least one of the insulating parts is provided with an insulating material such as an electrodeposition coating part or an insulating tube separate from the insulating resin for insert molding. The separate insulating material is provided so as to cover a given area of the outer circumference of the bar electrode or a given area of an outer circumference of the tubular electrode.