A fluid sterilization device includes: a processing flow passage in which a fluid subject to sterilization flows； a window member provided between a light source for irradiating the fluid in the processing flow passage with ultraviolet light and the processing flow passage, the window member including an incidence surface on which the ultraviolet light from the light source is incident and an exit surface from which the ultraviolet light exits toward the processing flow passage； and a heating body provided to be in contact with the incidence surface and adapted to heat the window member.
