PURPOSE: To enable the stable holding of a plowing depth at the set value in spite of the sinking of a machine body into a field.

CONSTITUTION: A rotary tiller 3 is installed so as to freely lift and lower relatively to a machine body 1 and a ground contact member 14 in contact with a tilled surface is provided on the side of an extended end at the rear of a tine shaft 9 of a rotary tiller 3 and connecting members 13 pivoted rockably around the shaft center concentric with the shaft 9. A controlling means 16 for performing the operation to lift and lower the rotary tiller 3 so as to change relative vertical positions relatively to the machine body 1 of the rotary tiller 3 based on results of sensing with an angle sensor 15 for sensing the rocking angle of the connecting members 13 relatively to the shaft center.