PURPOSE: To facilitate the change of discharge by providing a flow control valve for changing a first drain passage communicated with discharge passages, and a variable throttle means for changing the opening area of a second drain passage communicated with a back chamber.

CONSTITUTION: A radial plunger pump is provided with a fixed throttle 27 provided at discharge passages 25a, 25b, 'and a flow control valve 40 for changing the opening area of a first drain passage 37 communicated with the discharge passages 25a, 25b according to differential pressure between the pressure upstream of the fixed throttle 27 and the pressure of a back chamber 41. The radial plunger pump is also provided with a lead-in passage 38 for leading the pressure downstream of the fixed throttle 27 into the back chamber 41 of the flow control valve 40, and a variable throttle means 42 for changing the opening area of a second drain passage 39, communicated with the back chamber 41, from the outside. The desired discharge can be thereby obtained by operating the variable throttle means 42 from the outside. As a result, the discharge can be changed easily, and the cost related to the change of discharge can be reduced.