Unsteady cavitation that results from the non-uniformity of the flow field renders evil influences on marine propellers. It has been a general experiment for the investigation on such problems to observe unsteady cavitation in a flow field analogous to a wake pattern. At the time being, however, quantitative information cannot be derived from such kinds of experiment, and explanations of unsteady cavitation are likely to be qualitative, which can sometimes give us erroneous conclusions. The most basic and fruitful quantity for the study of cavitation is the pressure distribution on a unsteady hydrofoil in non-cavitating condition. With this quantity we can clarify unsteady cavitation more rigorously. This paper deals with two-dimensional hydrofoils pitching simple-harmonically. With this simplification pressure distributions can be directly measured, and can be calculated by the use of unsteady wing theory. These pressure characteristics have intimate relations with the real cavitation phenomena that were observed by high-speed motion pictures. Principal conclusions obtained are as follows. 1) The oscillatory flat-plate-wing theory shows good agreements with experiments when a rigorous correction is made for the steady component. 2) Characteristics of cavitation on a pitching hydrofoil can be explained by the time-dependent pressure distribution calculated by the unsteady wing theory. 3) Unsteadiness of pressure field strongly affects the process of the collapse of cavity. Increase of unsteadiness will lead to serious erosion. 4) Maximum length of unsteady cavitation on two-dimensional pitching hydrofoils tends to be shortened with the increase of reduced frequency up to 1.0.