To provide a soil improvement device by which lap work of a joint part with an existing improvement body can be carried out easily, surely and in a short time.

The soil improvement device 10 comprises an endless chain 40 having a plurality of excavation bits 41 fitted thereto, a guide frame 30 to which the endless chain is rotatably mounted, a nozzle 50 mounted on the guide frame to inject an improvement material W, a supporting frame 20 supporting the guide frame to be movable in up and down directions, and a running vehicle 22 to which the supporting frame is fitted. After the endless chain is inserted into the ground 7 by moving the guide frame 30 down, the running vehicle is made to run and the endless chain is rotated. A channel M is dug and the improvement material injected through the nozzle and the in-situ soil 8 of the ground are mixed together and stirred by the excavation bits of the rotating endless chain to form an improvement body 9 in the ground. In the soil improvement device 10, the nozzle 50 is swingably mounted on a front end of the guide frame 30.