PURPOSE: To compact a planting transmission case by adopting a speed change operation structure having a specific gear-selecting means with an engaging key.

CONSTITUTION: An operation shaft 19 is slidably inserted into the cylindrical shaft part at the end of a transmission shaft 10. An outwardly projected engagement key 21 is disposed on the operation shaft 19 through a long hole 20 long in the axial direction of the cylindrical shaft part. A key groove 22 is engraved on speed-changing gears G1-G4 loosely fit onto the cylindrical shaft part, and the operation shaft 19 is slidden with a member 23 to be operated. By the sliding operation of the operation shaft the gear intermeshed with the engagement key 21 is selected, thereby permitting to execute the speed change operation of the speed change gear device for transversely carrying seedlings.