To effectively suppress segregation of soil in treated soil until the treated soil falling down a feed pipe reaches the bottom by preventing a backflow of water into the feed pipe, in a treated soil placing device for feeding down fluid treated soil where solidifier is added to and mixed with soil from a placing ship to the bottom through a feed pipe to place it over the bottom.

The feed pipe 12 has an opening/closing valve V capable of opening and closing an outlet 12o of the pipe 12. To the opening/closing valve V, urging means F are coupled to apply urging force in a valve closing direction to hold it in a closed valve position Va. With increasing valve opening force exerted on the opening/closing valve V by the weight of treated soil S in the feed pipe 12, the opening/closing valve V is opened against the urging force of the urging means F.