Disclosed are a microbe inactivation processing method that can perform inactivation processing of microbes, while damage to human body cells is prevented or inhibited, with an efficient use of light emitted from a light source and the obtainment of a large effective irradiation area. Also provided are a cell activation processing method that can reliably activate target cells with high efficiency. The microbe inactivation processing method includes: a step of applying light emitted from a light source through an optical filter, with the light source configured to emit light having a wavelength within a wavelength range of 190 nm to 237 nm, in order to perform inactivation processing of a target microbe. When the light emitted from the light source is incident at an incident angle of 0°, the optical filter transmits at least a part of ultraviolet light having a wavelength within a range of not lower than 190 nm and not more than 230 nm, and transmits at least a part of ultraviolet light having a wavelength within a range of more than 230 nm and not more than 237 nm, and the optical filter blocks transmission of ultraviolet light having a wavelength out of a wavelength range of not lower than 190 nm and not more than 237 nm.