It is an object of the present invention to provide a method and an apparatus capable of selectively damaging and killing tumor cells. In one aspect of the present invention, there is provided a method for selectively damaging and killing tumor cells comprising a step of irradiating tumor cells with a pulse light having continuous emission spectra ranging at least from 230 to 270 nm (UV pulse flash), outside a living body of a human or a living body of a non-human animal or in a living body of a non-human animal. The UV pulse flash, preferably, has an accumulated irradiation amount per unit area that is achieved at a distance of 8 cm from a light source having an integrated output of, for example, 90, 180-7100 or 14200 J. In other words, the UV pulse flash, preferably, has an accumulated irradiation amount per unit area of, for example, 6, 12-480 or 960 J/cm 2 in terms of an energy originated from a wavelength of UVC. The step of irradiating with the UV pulse flash is preferably carried out, for example, within 1 minute. The UV pulse flash is preferably emitted from a xenon flash lamp. As another aspect of the present invention, there is provided an apparatus for treating tumor tissues comprising a light source of the UV pulse flash.