The authors are studying a diagnostic method of a PV power generating system. We consider that the change of I-V characteristics can be utilized for the diagnosis. However, the report on the change of I-V characteristics is very little. In this paper, we investigate the relation between the output lowering due to shaded PV cells and the change of I-V characteristics, utilizing the computer simulation. It was proven from the simulation that I-V characteristics are changed by the condition of the shadow, which covered the module. The change of I-V characteristics of a PV module with shaded PV cells is discussed by the shift of the avalanche breakdown voltage of shaded PV cells. (C) 2002 Elsevier Science B.V. All rights reserved.