The present invention provides a surface treatment method that improves antimicrobial activity of copper or a copper alloy and enhances immediate effects of antimicrobial actions on the surface of the copper or the copper alloy. A surface treatment method for copper or a copper alloy according to the present invention comprises preparing a reducing agent solution containing a biological reducing substance, and treating the surface of the copper or the copper alloy with the reducing agent solution. The present invention also provides a surface treatment liquid for sterilizing copper or a copper alloy, in which the surface treatment liquid contains a biological reducing substance. The present invention also provides a sterilization method that comprises bringing copper or a copper alloy treated by the surface treatment method into contact with a surface of an object to sterilize the surface of the object.