An application of contra-rotating rotors, in which a rear rotor is employed in tandem with a front one and these rotors rotate in the opposite direction each other, has been proposed against a demand for developing higher specific speed axial flow pump. The most important feature of contra-rotating axial flow pump is that the rear rotor, installed instead of the stator of conventional type, plays a role not only to recover the static head but also to give the energy to the fluid directly. Then, the rotational speed or the pump size could be reduced with the same design specifications of pump head and flow rate as those of the conventional type. As another advantage, contra-rotating rotor theoretically yields a stable head-characteristic curve with a negative slope. In actual performance test of contra-rotating axial flow pump, a positive slope of head-characteristic curve, however, appears at the partial flow rate, which seems to be in the same manner to conventional type, depending on the rotors design. In the present study, in order to investigate the flow mechanism of head deterioration at the partial flow rate, blade-to-blade velocity distributions around the rear rotor have been measured with Laser Doppler Velocimetry (LDV) in the partial flow rate conditions by ensemble averaging technique with the rotating position of the rear rotor. Flow distributions in the tip region of rear rotor at flow rates just before and after the head deterioration are compared and the flow mechanism of head deterioration is discussed with the help of static head distribution on the casing wall.