A ridge formation method includes the steps of: providing a sieve instrument immediately after a tilling unit configured to scrape soil to throw the soil backward while rotate a rotor having a plurality of soil scraping pieces to cause the tilling unit to travel, the sieve instrument configured to receive the thrown soil to perform a sieve operation and moving the sieve instrument in a reciprocating manner in a traveling direction to generate an impact on the received soil so that the soil is finely grained, and to separate the finely-grained soil from other impurities, whereby the finely-grained and separated soil is allowed to pass through a sieve mesh and fall onto a ground so that the falling soil is accumulated on the ground so as to form the ridge substantially the same in width as the sieve instrument on the ground.
