A HST system (1) for use in traveling of a vehicle includes a controller (51) that controls a pump regulator (23) that changes a displacement of a pump (22) and a motor regulator (25) that changes a displacement of a motor (24). The controller (51) determines whether or not a particular downhill travel condition is satisfied based on a result of detection by a vehicle speed detector (62) that detects a vehicle speed of the vehicle and a result of detection by a rotation number detector (61) that detects a number of rotations of an engine (21) per unit time. In a case where a depression amount of an accelerator pedal (52), which is detected by a depression amount detector (63), is zero and the particular downhill travel condition is satisfied, the controller (51) controls the motor regulator (25) to increase the displacement of the motor (24).
