This paper presents a 3-dimensional seismic isolation floor developed to protect computer systems in nuclear power plants from strong earthquakes. The 3-dimensional seismic isolation floor consists of floor panels, a floor frame, horizontal coil springs, isolation-supporting devices and devices for controlling rocking motion. In order to examine the performance of the 3-dimensional isolation floor, vibration tests of an isolation floor model, on which an actual computer system was set up, were carried out using a 3-dimensional shaking table. Test results confirmed that the actual computer system was effectively isolated by the 3-dimensional seismic isolation floor.Moreover, an analytical model was validated against these test results. For the analytical simulation the floor frame was representdd by five lumped masses and the isolation device was modeled as 2-dimensional springs and dampers. Both the calculated acceleration time history and floor response spectrum agree well with test results.