PURPOSE: To mix two kinds of fluids at a specific ratio without pulsation by making the composite waveform of the respective flow waveforms in two pumps constant, and determining the cam shape for one other pump in such a way that the flow waveform in this pump satisfies a specific condition.

CONSTITUTION: A nonpulsating pump 1 is provided with a first and a second pump on the suction side and a third pump on the discharge side, and the respective plungers 5-7 of these pumps are driven in reciprocating motion by a rotary shaft 21 through the respective cam mechanisms 18-20. In this case, the suction- discharge flow waveforms q1, q2 of the first and second pumps 2, 3 are respectively set to the specific value, and the composite waveform q3 thereof is set to A0sinθ, where A is plunger cross-sectional area, is eccentric quantity, and α is a cam rotating angle. The shape of the cam 49 of the third cam mechanism 20 is then determined to satisfy q4=A0(1/π-sinθ) when the suction-discharge flow waveform q4 of the third pump 4 is 0≤θ≤π and to satisfy q4=Aθ0 (1/θ) at the time of π<θ<2π.