Method of improving interior environment and grounding electrode device

To maintain an interior environment in an environment in which negative ions are abundant, and to stably maintain the environment at a higher level with maintaining an efficacy of the charcoal included in a paint containing a charcoal powder applied to a wall face in a room, a floor face in the room is formed by an insulator and the wall face is made to have a grounding electric potential by connecting it to a grounding electrode buried in a ground according to a method of the invention. The grounding electrode device includes a main body formed in a bottomed tubular body by a metal, and a charcoal and a catalyst are accommodated in the main body.