Friction between pipe and support structure is generally known to reduce seismic response of the piping systems. Vibration tests using large-scale piping model with friction support were carried out to evaluate the reduction effect. The piping response was mainly governed by the first modal deformation. The simplified analysis method based on linear response spectrum analysis was developed and confirmed to be applicable. In this method, the reduction effect by friction is treated as equivalent viscous damping ratio. This paper deals with the analysis method, and the comparison between the experimental results and analytical ones.