When the load acting on a fluid motor (25) increases, a changeover valve (61) starts to be changed over toward a low-speed position L, but at this time an additional pilot fluid whose pressure is determined by the amount of fluid flowing into or out of a second passage (68) through first and second notches functions as a pressure regulator for a spool (65), and lowers the pressure within the second passage (68) without lowering the pressure within a high pressure-side main circuit (28), thereby tilting a swash plate (26) from a position of high-speed rotation to a position of low-speed rotation.