A travel control unit of a working vehicle, comprising: a hydraulic continuously variable transmission which is interposed between an engine and a wheel, which includes a hydraulic pump driven by the engine and a hydraulic motor fluidly connected to the hydraulic pump, and wherein the hydraulic pump and the hydraulic motor are respectively of a continuous displacement variable type in which a displacement continuously varies; an operator sensor which detects a position of an acceleration operator; and a controller which changes the displacement of the hydraulic pump based on the position of the acceleration operator; wherein the controller always sets the displacement of the hydraulic motor at an initial stage of starting to travel to a maximum displacement in a variable displacement range; the travel control unit of working vehicle being characterized by a vehicle velocity detecting unit which detects a vehicle velocity, and the controller being characterized in that it stores an efficiency map defining a highest efficiency point of the engine corresponding to a load value, the capacities of the hydraulic pump and the hydraulic motor, and a vehicle velocity, in that it obtains, at stages other than the initial stage of starting to travel, the capacities of the hydraulic pump and the hydraulic motor at the highest efficiency point based upon the load value, the vehicle velocity detected by the vehicle velocity detecting unit and the efficiency map, and in that it controls the hydraulic pump and the hydraulic motor at stages other than the initial stage of starting to travel to realize the capacities of the hydraulic pump and the hydraulic motor at the highest efficiency point. a hydraulic continuously variable transmission which is interposed between an engine and a wheel, which includes a hydraulic pump driven by the engine and a hydraulic motor fluidly connected to the hydraulic pump, and wherein the hydraulic pump and the hydraulic motor are respectively of a continuous displacement variable type in which a displacement continuously varies; an operator sensor which detects a position of an acceleration operator; and a controller which changes the displacement of the hydraulic pump based on the position of the acceleration operator; wherein the controller always sets the displacement of the hydraulic motor at an initial stage of starting to travel to a maximum displacement in a variable displacement range; the travel control unit of working vehicle being characterized by a vehicle velocity detecting unit which detects a vehicle velocity, and the controller being characterized in that it stores an efficiency map defining a highest efficiency point of the engine corresponding to a load value, the capacities of the hydraulic pump and the hydraulic motor, and a vehicle velocity, in that it obtains, at stages other than the initial stage of starting to travel, the capacities of the hydraulic pump and the hydraulic motor at the highest efficiency point based upon the load value, the vehicle velocity detected by the vehicle velocity detecting unit and the efficiency map, and in that it controls the hydraulic pump and the hydraulic motor at stages other than the initial stage of starting to travel to realize the capacities of the hydraulic pump and the hydraulic motor at the highest efficiency point.