In this paper the authors describe investigations of the detailed cavitation structure, those are cavity length and its fluctuation, cavity shape and volume, void ratio in the cavity and velocity and pressure distribution around a cavitated foil. The following six foil sections were tested : Non-burst type : NACA 0015, NACA 16-015, NACA 4415 Burst type : MAU 0.6r, MAU 0.9r, 4% thickness Plano-Convex Results are as follows : (1) At a bursted foil, a vortex cavitation occurs first in the shear layer of the leading edge separation zone, when the cavitation number is greater than- C_{P min} of the foil. This vortex cavitation is very similar to the cloud cavitation. Decreasing pressure, it changes to the ordinary sheet cavitation. (2) Sheet cavity is trapped in the separated zone, which does not coincide with the lowest pressure zone. Consequently a region, whose pressure is lower than vapour pressure, does exist at the upstream of the sheet cavity on a foil. (3) Cavitation characteristics of foils can be classified into two categories according to whether the laminer separation bubble will burst or not. Cavity length of a bursted thin foil can be calculated by the linear closed cavity model theory. On the contrary, the cavity length of a non-bursted foil agrees with the result of “lift equivalent method” (empirical method). (4) Cavity volume can also be classified by the condition of burst or non-burst. Empirical formulas are proposed from the present experiment.