To provide a roadbed improvement method that achieves simplification and a reduction in work burden by enabling formation of an improved-soil roadbed stabilized by solidifying a soft roadbed under a track, deformed and sludged by fluidization caused by rainwater and the like.

In a roadbed improvement method, when ballast is backfilled in a ballast-partially-removed location after the partial removal of the ballast under a railroad track, the ballast for backfilling is partially substituted with a hydraulic composition such as a cement clinker. Thus, the hydraulic composition is crushed or worn away, dropped to a roadbed and mixed into soil forming the roadbed, and solidified by water contained in mud in the roadbed and water such as rain water, so as to improve the soil of the roadbed.