A fluid sterilization device includes: a treatment flow passage in which a fluid subject to sterilization flows； a light source chamber that stores a light source for irradiating the fluid in the treatment flow passage with ultraviolet light； a window member that is provided between the treatment flow passage and the light source chamber and transmits ultraviolet light； and a pressure adjustment mechanism that adjusts a pressure in the light source chamber so as to reduce a difference between a pressure in the treatment flow passage and the pressure in the light source chamber.
