Rotating cavitation is one of the flow instabilities associated with cavitation, which has been a problem for the development of turbopumps of liquid rocket engines since it causes super-synchronous shaft vibration. Recently, it is argued that rotating cavitation might also cause large repetitive stresses on blades. This paper describes about the theoretical prediction of unsteady blade forces on the cavitating inducer under rotating cavitation. Unsteady pressure distributions and a lift force for a 2-dimensitional flat plate cascade under rotating cavitation are calculated by using a linearized free streamline theory based on the unsteady closed cavity model and singularity method. Since the linearized calculation cannot predict the absolute magnitude of the fluctuations, the unsteady pressure distributions and resulting lift fluctuations are presented for a given magnitude of cavity length fluctuations.