To dispose of excavated sediment containing deleterious material without making it open to the air.

This disposal method for excavated sediment includes: an excavating process of digging a tunnel 1 by an excavator 2; a discharge process including a loader 4, a first conveyer 3, a second conveyer 5, a third conveyer 8 and a carrier 7 for discharging the excavated sediment S from the tunnel 1; and a disposal process for mixing deleterious material processing agent T and the sediment S by a mixer 6 to disposal of the deleterious material of the excavated sediment S. In the disposal process, the mixer 6 is provided between the second conveyer 5 and the third conveyer 8 in the discharge process to thereby dispose of the deleterious material. The deleterious material processing agent T is mainly composed of unburnt peridotite. Thus, the deleterious material contained in the sediment S can be dissolved, made insoluble and purified in the tunnel 1, so that atmospheric release of the deleterious material to the outside of the tunnel 1 can be reduced as much as possible.