Ejector pumps used for dredging at the bottom of deep sea is usually operated with the sand-pumps. The author presents here in this report the efficiency of series operation of ejecor pump and sand-pump as the first step of this research through experiments. Each diameter of suction and delivery pipe of the ejector pump used for this experiment is 80.7mm and has two nozzles on its side. And the diameter of centrifugal pump used for this experiment was 80mm. As for the series system of ejector and centrifugal-pumps, operating points of ejector are on the pressured characteristic curve of ejector, and the operating points of pump are usually on the characteristic of Q-H curve.
　So operating points of this whole pipe system are usually determined by the characteristic of ejector and centrifugal pump.
　In order to make the design of these pipe system easy, the author showed here in this rports calculation for the operating points at the series operation of ejector and centrifugal pump, and also compared them with experimental results.
　The effect of throttle of part of ejector and the change of characteristic of it with the size of nozzle diameter were also confirmed through experiments.
　By these experiments, optimum throttle are found and following tendency was also confirmed that with the elevation of oiperation pressure of ejector, the head of whole system of pipe are raised, while as for the operating points of centrifugal pumps, flux increases, and head become lower, which need more power for the centrifugal pumps.