It began when the rice tithed to the central government as Soyocho under the Ritsuryo system (a system of centralized government based on the ritsuryo code) went into arrears in the Heian period, and Denso that was kept as Fudokoku (emergency grain) was tithed to the central government for use in food supplies for government officials.
律令制において租庸調として中央に貢進されていた米が平安時代に滞るようになると、官司の大粮に宛がうために本来は不動穀に充てられる筈であった田租を中央に貢進したのが最初とされている。