Improving the ground beneath or beside existing structures is one of the most effective countermeasures against structural damage caused by seismic motion of the ground. Economical and mobile methods will be in high demand in Japan where seismic events with greater intensity are anticipated in the near future. The authors have developed a new in-situ soil mixing method using collapsible/expandable mixing blades and a special monitoring/auto-control system. This paper first describes the new technique in comparison with conventional soil mixing methods and describes the results of field tests. Lastly, an actual application is described in which a slope of loose deposits filling a valley is improved with inclined soil columns.