With the aim to clarify the effects of leading edge sweep on unsteady cavitations, experiments were carried out on 4-bladed inducers with forward/backward leading edge sweep. It was found that, with the leading edge sweep, mean cavity length was shorter than that without sweep at the same flow rate and cavitation number. The ranges of alternate blade cavitation and unsteady cavitations, such as rotating cavitation and cavitation surge, were shifted toward lower cavitation number. In addition, it was found that the onset of unsteady cavitation depends on the steady cavity length. A simple scaling rule for steady cavitation development, first proposed by Ihara et al. (1989) for a single hydrofoil, and developed independently by Acosta et al. (1999) for realistic inducers, has been applied to the present cases. It was found that the rule reasonably explains the effects of sweep on steady cavity development. Since the stability of cavitating flow depends only on the steady cavity length, it was confirmed that the scaling rule can also explain the effects of sweep on various kind of cavitation instabilities.