This paper has experimentally investigated transient voltages on floors in an intelligent building due to lightning with special reference to grounding of a lightning rod, the building and the floors by using an 1/10 scaled-down building model. It has been found that the floor voltages are the smallest in the case of the floors bonded to the building structure except the case of the structure being used as a lightning rod. The common grounding of all the floors causes very high floor voltages when the floor grounding is connected to the lightning rod grounding. Individual grounding of the floors, the building and the lightning rod produces rather high voltages on the floors. A voltage difference between the floors is significantly reduced by bonding the floors to the building structure or by the common grounding of the floors.