A phototherapy device which obtains an advantageous therapeutic effect and suppresses side effects at least to a certain degree by irradiation of diseased sites with therapeutic radiation with a spectrum in the UV-B radiation wavelength range that is continuous at least in the wavelength range of at most 303 nm with a lower boundary wavelength value of at least 297 nm. The phototherapy device has a light source which emits light with a spectrum having an emission peak in the wavelength range from 300 nm to 315 nm and which is continuous in the wavelength range of this emission peak at least up to 295 nm; and a radiation emission window in which the light from the light source is incident and via which the therapeutic radiation is emitted with a spectrum with a lower boundary value in the wavelength range from 297 nm to 303 nm.