It is expected for tracked vehicles to travel up to 60km/h in 2010. Tracked vehicles mainly run on the irregular road surfaces. Irregularity of road surface can make severe vibration of chassis. Excessive vibration of chassis deteriorates ride comfort, shooting accuracy during traveling, road holding and life span of electronic equipment. Therefore, to improve these performances, preview control is used. Because semi-active suspension can make a limited performance, active suspension unit will be employed. In previous research, preview control was applied assuming constant speed of vehicles. However, to apply preview control to the vehicle practically, it has to make a satisfactory performance under speed varying condition. To do so, preview control which can be applicable under speed varying condition will be proposed.