Characteristics of low-frequency pressure waves radiated from a train passing a Shinkansen viaduct were examined from field measurements based on the manual for measuring low-frequency noise issued by Ministry of the Environment Government in October 2000, using a low-frequency sound level meter (NA-18 A) manufactured by Rion Co. Ltd. Measurements proved that low-frequency noise comprises a hydrodynamic pressure variation and an acoustic pressure wave. The hydrodynamic pressure variation is designated as the train passing pseudo-sound and the acoustic pressure wave is designated as the low-frequency pressure wave. The low-frequency pressure wave is emitted from a line source and is proportional to the 4th-5th power of train velocity and inversely to the first power of the distance from the source to the observation point.