A hydraulic transmission (21) has an electric actuator (86) for changing its output/input rotation speed ratio. A controller (90) of the actuator memorizes a command current value to the actuator supposing that load is not applied on the hydraulic transmission, and calculates a value of load applied on the hydraulic transmission by calculating a difference between an actual command current value to the actuator and the memorized command current value. A command current value is compensated based on the value of load, and the actuator receives feedback to be controlled. A vehicle may have a differential mechanism (10), which combines rotation powers on input side and output side of the hydraulic transmission. A detector (82) detects rotation speed of a main speed change output shaft (27) serving as an output shaft of the differential mechanism. If the detected rotation speed differs from target rotation speed of the shaft (27) determined based on operation of a speed change operation tool, the actuator is forcedly controlled. This difference is supposed to be caused by load applied on the hydraulic transmission, whereby the load is calculated.