To create a soil improvement body with high quality in which fibers can be mixed in a ground improvement area with a predetermined mixing ratio in a soil improvement body creating method using a high pressure injection agitation method which mixes fibers.

In a soil improvement body creating method, fiber charging holes 12 are drilled in a plurality of positions around a rod insertion hole while being separated at a predetermined distance from the rod insertion hole in a ground improvement area P, aside from the rod insertion hole 11 drilled by a high pressure injection agitation method. Fibers 3 are embedded or fixed in the fiber charging holes 12 and a solidification material 4 is injected to the fiber charging holes 12 in the ground by high pressure by an injection nozzle 2 of a multiple pipe rod 1 inserted in the rod insertion hole 11, thereby agitating and mixing the fibers, the solidification material, and original ground.