A car-vibration analyzing system was developed for the maintenance of the riding quality of railway vehicles. This system, composed of a 16 channel FFT signal processor and a thermal printer, has a software program which evaluates car-vibration and riding quality. And it can process simultaneously 8 lateral vibration channels and 8 vertical vibration ones at maximum. The principal evaluation index is the Riding Quality Level LT which is a newly proposed index fundamentally based on ISO Standard 2631. LT is calculated form the power spectrum density of car-body acceleration analyzed by the FFT signal processor. The output data are divided into the following 3 groups and are displayed on a CRT. (1) Data for judging riding quality (2) Data for indicating improvement in performance (3) Data for predicting transition of vibration and riding quality in increase of running distance.