As the ballast crushing and refining progresses over time and the content of fine particles increases, the saturation of ballast increases during rainfall by decreasing in drainage, the strength decreases, and so settlement of ballast is likely to increase. At present, since there is no method for quantitatively evaluating the ballast condition at field spots, the ballast condition is visually checked, and the ballast renewal is performed in consideration of the track repair frequency. Therefore, we had investigated the condition of the ballast under different track repair frequencies in the real ballasted tracks and evaluated the particle size distribution of the ballast sample. And we had developed an inspection method using sound transmission for quantitatively evaluating the ballast condition. This method uses the characteristic that as the ballast progressed to be crushed and refined from a new condition, it becomes difficult to transmit sound in ballast. Therefore, we report that the relationship between sound level of transmitting ballast and ballast fouled index has been evaluated by the full-scale model tests. Moreover, we report on the results of the field tests to verify validity of this method.