A fluid sterilization device 10 includes: a treatment flow passage 12 in which a fluid subject to sterilization flows； a light source chamber 14 that stores a light source 30 for irradiating the fluid in the treatment flow passage 12 with ultraviolet light； a window member 32 that is provided between the treatment flow passage 12 and the light source chamber 14 and transmits ultraviolet light； and a pressure adjustment mechanism 40 that adjusts a pressure in the light source chamber 14 so as to reduce a difference between a pressure in the treatment flow passage 12 and the pressure in the light source chamber 14.
