An attachment of inducer is a powerful method to improve the cavitation performance of a turbo-pump. Cavitation surge phenomena, occurring under the severe suction pressure at a partial flow rate, is focused in the present paper. Flow measurements were carried out at the inlet and outlet sections of a flat-plate helical inducer with the solidity of 2.0 and tip blade angle of 11° by using a total-head yaw-meter with a phase locked sampling method in one period of the shaft rotation or the cavitation oscillation. Time variation of the flow distributions during the oscillation is clarified with observed cavitation behaviors. After these results are compared with those in conditions just before and after the oscillations, the fluctuating blade forces are estimated from the blade to blade distributions of casing wall pressures measured in these conditions.