To provide a soil improving device capable of reducing the removing work of the stuck surplus soil, improving the reutilization factor of the improvement object soil, and reducing the discard quantity of soil.

This soil improving device is provided with a preprocessor 11, a kneading device, and a lime mixing device. The preprocessor 11 mixes a water absorbing agent 23a absorbing the free water included in the improvement object soil made of a soil component into the raw material soil A to granulate the improvement object soil and fractionates the raw material soil A into the improvement object soil and foreign matters. The improvement object soil is made to hardly grow sticky by granulation. The improvement object soil granulated by the preprocessor 11 is fed to the kneading device by the first feeder 14. The kneading device mixes a soil improving agent fixing on the soil surface by absorbing the free water included in the soil into the fed improvement object soil to granulate the improvement object soil. The improvement object soil granulated by the kneading device is fed to the lime mixing device by the second feeder. The mixing device mixes quick lime into the fed improvement object soil to obtain the improved soil having fluidity and strength.