PURPOSE: To reduce the penetration of a thunder surge current into a building at the time of falling of a thunderbolt to a high structure.

CONSTITUTION: This is the grounding structure of a building installed on the rooftop of a building which lets a thunder surge current go to the earth via lightning conductors 9 at the time of falling of a thunder bolt to exhaust tubes 2 by connecting the lightening conductors 9 to exhaust tubes 2 installed at one or more places on rooftop. The lightening conductor 9 is pulled out to the outside of the external wall of a nuclear building 1, and it is connected with a pull-down conductor 12 provided outside, and also the pull-down conductor 12 is connected to the grounding system 3 laid around the nuclear reactor building 1. The pull-down conductor 12 leads a thunder surge current to the grounding system 3, and reduces the quantity of thunder surge current penetrating into the nuclear reactor building 1, and it has impedance not more than that of the external wall on the side where the exhaust tube 2 is installed. Hereby, the potential gradient within the building can be flattened, and the malfunction of facilities, damage, etc., can be reduced.