There are provided: a first circuit for traveling HC1, formed through a closed circuit connection of a variable displacement hydraulic pump 2 and a variable displacement hydraulic motor 3, that includes a motor control unit 10, 11 that controls a displacement of the hydraulic motor 3; a second circuit for working HC2 that drives a work hydraulic actuator 114, 115 by pressure oil from a work hydraulic pump 4; and a maximum value control unit that controls a maximum displacement of the hydraulic motor 3 in accordance with load pressure Pf in the second circuit HC2. The maximum value control unit decreases the maximum displacement to a minimum limit q1 corresponding to a maximum load pressure Pr in the second circuit when load pressure Pf in the second circuit exceeds a predetermined value Ps.