The internal flow in the centrifugal pump with a circular casing is measured by PIV method. The casing and impeller of semi-open type have two-dimensional shapes. The type number of the pump is 2.44×10^<-1> and specific speed is 100[m, m^3/min, min^<-1>]. The unsteady flow near the exhaust tongue is measured mainly. Even at the best efficiency point, the flow around the impeller is not symmetric. The radial velocity is remarkable only at the exhaust area of the casing. The flow near the impeller blade changed dynamically according to the relative position of the tongue.