To provide a surface treatment method of a bioelectrode, capable of retaining stable measurement accuracy for a long period without causing peeling of or lack of a black film formed by surface treatment on an electrode surface of the bioelectrode, despite of repetitive uses.

This surface treatment method is characterized in performing a first process for dipping silver electrode 5 in ethanol containing chloroauric acid, as a negative electrode, and applying a current between itself and a positive electrode, a second process of dipping the silver electrode 5 in ethanol containing urea, and a third process for dipping the silver electrode 5 in the ethanol containing chloroauric acid again, as the negative electrode, and passing the current between itself and the positive electrode, and treating the electrode face 5a of the silver electrode 5 with the black film.