PURPOSE: To prevent damage to a work machine at the time of inspection, repairing or cleaning by setting torque at the time of inverse rotation to be smaller compared to torque at the time of normal rotation.

CONSTITUTION: An HST transmission device is interposed in a transmission circuit for transmitting power to a PTO shaft 1 of a work vehicle for enabling the PTO shaft to rotate normally and inversely, and the maximum output horsepower when the PTO shaft rotates inversely is set to be smaller than the maximum output horsepower at the time of normal rotation so as to prevent breakage of the work vehicle. Otherwise, the HST transmission device is interposed inside a transmission circuit for transmitting power to the PTO shaft 1 of the work vehicle, and output limiting or input limiting is performed for the HST transmission device based on the result of computation of an input rotation speed or output rotation speed of the HST transmission device with which rotation of the PTO shaft 1 is changeable steplessly, and a hydraulic pressure of the HST transmission device.