PURPOSE: To enable to discharge the fluid continuously at a constant rate, by shaping a cam for driving pistons to have a developed surface in the from of a Sin2θ curve.

CONSTITUTION: Four pistons 4 are disposed equidistantly along the same circumference, and sinusoidal curves F1WF4 having a maximum fluid discharging velocity respectively at angles of 90, 180, 270 and 360 degrees are formed as shown at (a)W(d) in the drawing. By superposing these sinusoidal curves as show at (e) in the drawing, fluid is discharged at a resultant flow rate (f) since an equation Sin2θ+Cos2θ=1 is satisfied. That is, fluid is drawn into cylinders 2 by the movement of pistons 3 and discharged at a rate conforming to the differential curve of Sin2θ on its positive side at a velocity of pistons 3 determined by a cam 6. Therefore, the resultant discharge rate of fluid can be controlled to be always constant by combining these for sinusoidal curves.