Through the research on an air-mixed water injector for reducing the cavitation noise of a ship's propeller it had been noticed that the difference of the bubble radius between fresh water and seawater might have an influence on its radiated noise. In order to investigate the influence, an experimental apparatus was newly manufactured, and then the cavitation noise was measured with both fresh water and seawater. As a result of the measurement, it was found that the inception cavitaiton number in seawater was 15% lower than that in fresh water, and also that the noise spectra in fresh water had the same characteristics as those obtained at a 15% lower cavitation number in seawater. An ordinary simulation test with fresh water tended to overestimate the full scale propeller cavitation noise in the sea by about 20%. Simulating the motion of bubbles by the Rayleigh--Plesset equation by considering the surface tension, viscous effect, and the pressure inside the bubble, it was predicted that the ratio of gas content would cause the different characteristics of the cavitation noise between fresh water and seawater.