Dredged soil caused by dredging work has brought the limits of the capacity of sediment disposal facilities in Japan. One way to solve this problem is considered to efficiently reduce the dredged soil and secure the capacity of the sediment disposal facilities. As a method of volume reduction, the technique utilizing ultrasonic vibration for compacting of soil particles has been studied so far. There are two purposes of this study. One is to clarify the volume reduction characteristic of water dredged soil by ultrasonic wave irradiation by changing the frequency or output voltage. The other is to take aim at 30% of the volume reduction. In the experimental method, the slurry of either sand or silt, its moisture content adjusted to 150%, was poured into each container. Ultrasonic wave was irradiated from the horizontal direction. After the experiment, the mass of the sample was measured. Relative density and volume reduction ratio were calculated through wet density, dry density, and void ratio. As a result, the highest volume reduction ratio of 27% was obtained when the ultrasonic wave of frequency 28kHz with output voltage 200V were irradiated in the silt. In conclusion, although we did not reach the target volume reduction of 30%, we were able to confirm the increase in density and the decrease in volume.