In order to clarify the mechanism of cavitation cloud shedding, the shedding frequencies of cavitation clouds were investigated by high-speed observations and image analysis. The results indicate that the shedding frequency follows the similarity law for unsteady vortex flow. The ranges of parameters considered were from 0.4 to 2.0 mm for the nozzle throat diameter, from 10 to 30MPa for the injection pressure, and from 0.01 to 0.05 for the cavitation number. The similarity law is discussed with reference to the Strouhal number, which depends on the shedding frequency of the cavitation cloud, the width of the cavitating jet and the jet velocity at the nozzle exit. Formulas describing the relationships between the shedding frequency of the cavitation cloud and the investigated parameters were developed. In addition, the Strouhal number for cavitation cloud shedding was shown to be independent of the investigated parameters and constant, with a value of 0.18. In fact, the cavitation cloud shedding was governed by the constant Strouhal number.