Defects present on the Ge(111)-c(2 X 8) surface and an effect of surface electronic states on the defect-induced perturbation have been studied by scanning tunneling microscopy (STM). The defects exhibit voltage-dependent characteristics in the image. In particular, the empty-state images with low bias voltages exhibit delocalized brightness variation around the defects, suggesting that these defects are charged relative to the clean, unperturbed surface. The voltage-dependent but nonmonotonic amplitude of the delocalized brightness in the STM image is explained in relation to the surface electronic structure of Ge(111)-c(2 X 8) and the tunneling probabilities.