Ultraviolet (UV) irradiation is well known to induce apoptosis, a hallmark event of which is the occurrence of sunburn cells in the epidermis. Keratinocytes in which DNA damaged by UV irradiation is not repaired undergo apoptosis as sunburn cells. However, we have previously reported that low-dose UV-B irradiation (∼0.1 J/cm²) suppressed the apoptosis induced by cell detachment and serum depletion. Dysregulation of apoptosis is important in tumor progression and malignancy and in promoting resistance to cancer therapy. To develop a better understanding of the antiapoptotic effect of UV irradiation, and to design the effective induction of apoptosis, we tried the proteome analysis of the molecules regulating apoptosis in low-dose UV-B–irradiated NIH3T3 cells, using two-dimensional difference gel electrophoresis (DIGE). Of a total of 3811 protein spots detected, 42 were found to be different between the cells undergoing apoptosis and cells after the irradiation. Of the spots selected, 25 were identified using MALDI-TOF/TOF-MS, some as structural proteins. Although typical apoptosis-related molecules were not detected, possibly because proteins with low molecular weights were difficult to identify in the gel conditions used in this study, some of the proteins were considered to be involved in apoptosis. The DIGE system used in this experiment has advantages (including a high level of statistical confidence) for discovering new functional proteins related to the regulation of apoptosis.