To enable the efficient oxidative decomposition of organic chlorine compounds and the like in bottom mud with simple equipment; to return decontaminated bottom mud to a water area without using a special equipment.

A long cylindrical member 12 is immersed so that the top end opening 11 is located in the vicinity of the water surface 26 of the water area 24 and the bottom end opening 13 is located in the vicinity of the water bottom 28. Ozone gas is blown from an aeration pipe 14 disposed at the bottom of the long cylindrical member 12, and water flow from the outside of the bottom end opening 13 to the inside thereof is generated by the air lift effect by bubbles of the aerated ozone gas, thereby the bottom mud 32 is introduced into the long cylindrical member 12. The introduced bottom mud 32 rising in the long cylindrical member 12 is brought into contact with the aerated ozone gas.