For the purpose of clarifying the transient characteristics of a diffuser pump during start up, a series of tests was carried out in which three factors affecting the characteristics were varied. These factors were mass of water in the pipeline, valve opening and starting time. The results of the tests indicate that the locus of operating points on the head-capacity plane during pump start up deviates from the system resistance curve to a great degree as the mass of water in the pipeline becomes large and the starting time becomes short, and the trail of the operating points approaches the shut-off point. The results also reveal that the operating points follow generally the steady characteristics curve within the region where φ≥0.05 on the dimensionless head capacity plane. Analysis of the total pump system was made on the basis of one dimensional equations of motion using an analogue computer. The results of the analysis were almost the same as those obtained from the experiments. Axial thrust during start up was measured and is discussed also in this study.