For structural design of the floor slab concerning environmental vibration, a support system is developed on the widely diffused personal computer. Since the system is composed of the simplified analysis using the Rayleigh-Ritz method and the graphical user interface, the interactive operation of the system is actualized. Designers can obtain the analysis results promptly and visually to check the design. Moreover, the sensitivity analysis can be used for design changes. By using this system in comparison with measured results, designers can easily accumulate the important know-how about the floor vibration analysis. The know-how is of help for next design.