A gel layer retained in a retaining member 15, such as non-woven fabric, is placed on one side of an electrode element 1. The electrode element 1 is held by a flexible and extendable tape 11, and the tape 11 adheres to a biological surface tissue. When being plastered onto the biological surface tissue, the tape 11 is covered with cover films 12 and 13, thereby preventing occurrence of a kink in the tape 11. After the tape 11 has been attached to the biological surface tissue, a butt-joined portion between two sub-divisions constituting a cover film 12 is exfoliated to the outside, thus preventing removal of the tape 11.