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In order to investigate the pressure fluctuation induced by an unsteady cavitation on propeller blades, the unsteady cavity is replaced by a spherical bubble moving with varying radius. Then the pressure fluctuation induced by the sphere is calculated. The calculated results show that the second derivative of cavity volume with respect to time ∂^2V/∂t^2 constitutes the main contribution to the pressure fluctuation induced by unsteady cavitation of propellers. The solid boundary factor representing image effect of the hull surface is also calculated. It is then proved that the factor is approximately 2.0. Finally, it is shown that the pressure fluctuation measured on a flat plate above a cavitating propeller can be interpreted by the present theory.