PURPOSE: To obtain a controlling structure using an HST having excellent operability and various advantages such as smooth transmission behavior and the ability of normal and reverse rotations by contriving a transmission system to inhibit the transmission of an engine power to a running gear and a transplantation apparatus when the speed of the transplanter is slower than a prescribed level to cause floating of seedlings.

CONSTITUTION: A rice-transplanter for transmitting an engine power through a main clutch 10 to an HST 12 for transmitting the variable speed output to rear wheels and a transplantation apparatus is provided with a means A for detecting the working state of the transplantation apparatus, a means B for detecting the operation position of the HST 12, a means C for connecting and disconnecting the main clutch 10 and a controlling apparatus D for interlocking the working state detection means A, the position detection means B and the clutch operating means C in such a manner as to inhibit the coupling of the main clutch 10 when the transplantation apparatus is in working state and the HST 12 is in a forwarding range at a speed lower than the lowest speed to enable suitable transplantation work.