Continuous-wave THz radiation (0.10 THz and 0.28 THz) was irradiated to HeLa cells treated by bleomycin (BLM), and the induction/repair of DNA damage was observed by visualizing γH2AX. The signal intensity of γH2AX was decreased during and after the BLM treatment when 0.28 THz wave (250 mW/cm 2 ) was irradiated to the sample, suggesting that the THz wave enhances DNA damage repair. This phenomenon was not reproduced by 0.10 THz continuous-wave irradiation (200 mW/cm 2 ), suggesting the irradiation effect’s frequency dependence. Also, we performed a thermal control experiment by heating the sample to 42°C and observed the increase of γH2AX signals, indicating that the 0.28 THz irradiation affects DNA damage repair in a temperature-independent manner.