To provide a soil improving device reducing the kneading deficiency of a soil improving agent into soil and capable of simply kneading them.

This soil improving device is provided with a preprocessor 11, a first feeder 14, a kneading device 12, a second feeder 15, and a lime mixing device 13. The preprocessor 11 fractionates the raw material soil into the improvement object soil made of a soil component and foreign matters in the raw material soil. The kneading device 12 fed with the improvement object soil by the preprocessor 11 via the first feeder 14 kneads the soil improving agent fixed on the soil grain surface by absorbing the free water included in the soil into the improvement object soil to granulate the improvement object soil. The solution type soil improving agent such as a polyacrylic amide solution is used for kneading to realize kneading with little kneading power, and kneading deficiency is reduced. The lime mixing device 13 fed with the granulated improvement object soil from the kneading device 12 via the second feeder 15 mixes quick lime into the improvement object soil to obtain the improved soil having fluidity and strength.