In order to clarify the Johnsson effects which so often arise in single-foil cavitation-experiments, careful cavitation observations were made around a typical Clark Y foil of 4 percent thickness in our closed type cavitation tunnel, with which a number of cavitation-performance-tests have been done by late Prof. F. Numachi, for various cavitation numbers and incidences. For positive incidences of technical interest, we mainly observed spot cavitation, which seemed to be very insensitive to the cavitation nuclei, as well as flow fluctuations, accompanying the vortex type-, the band type-, the large spherical-bubbles as well as the bubble cloud. Abrupt separations on large groups of spot cavitation bubbles were also periodically observed within the transient region.