When ballasted tracks include a high-mix ratio of fine particles, track irregularities tend to increase soon after track maintenance performed by tamping because the density change of the ballast is large, and such work impairs the density of the ballast. Therefore, in this study, we developed a method to fill gaps under sleepers with a filling material for the realignment of tracks without impairing the density of the existing ballast. The authors also performed testing to confirm the validity of the method. The results confirmed that this technique is an effective approach for the repair of ballasted tracks.