This paper gives a general description of the Marine Propeller Cavitation Tunnel which was built at the Department of Naval Architecture, the University of Tokyo in March, 1980. First the basic design concept on a cavitation tunnel is stated with emphasis on the importance to keep air content of water adequately. The tunnel has two interchangeable test sections. The propeller test section has a square cross section of 0.45m×0.45m. Maximum water velocity in this section is 11.2m/s. Another test section, the foil test section has a rectangular cross section of 0.60m×0.15m. Its maximum water velocity is 19.5m/s. Minimum and maximum absolute pressures in both sections are 0.1 and 3.0 bar, respectively. A series of cavitation test on a model propeller was made at various revolutions, static pressure and air content to demonstrate the importance of air content control. The measurement of tiny air bubbles, which act as cavitation nuclei, was also performed by a scattered light counting instrument.