PURPOSE: To enhance reliability and efficiency by loading a cutter chain on a vehicle which travels on rails and transferring digged ballast and spraying high pressure water to the transferred ballast and eliminating nonconforming fine particles and collecting them.

CONSTITUTION: A ballast exchange device 21 is inserted into a ballast 11 in the lower part of a sleeper 12 where a vehicle 22 is forced to move forward along rails 13 by driving and circulating a cutter chain of the device 21. The cutter chain diggs the ballasts 11 and transfers the digged ballast to a selection chamber installed to the rear side of the device 21 as well. High pressure water is blasted to the ballast where nonconforming fine particles are transferred to a collection chamber of the device 21 by way of a slit and collected and eliminated with a vacuum hose. The conforming ballast 11 remaining in the selection chamber is further transferred backward with the cutter chain and returned downward to the sleeper 12. This construction makes it possible to prevent the mixture of the nonconforming ballast and dig and select ballast only with a single process and reduce the size without installing a classifier separately.