A fluid sterilization device includes: a housing formed with a processing passage for sterilizing a fluid passing through the processing passage; a light source that radiates ultraviolet toward the processing passage; and a transmissive member provided between the light source and the processing passage and transmitting the ultraviolet light radiated by the light source. The transmissive member is a one-piece component including: a focusing part that focuses the ultraviolet light radiated by the light source toward the processing passage by refracting or reflecting the ultraviolet light; an exit part that guides the ultraviolet light transmitted through the focusing part toward the processing passage, and a supported part formed on an outer circumference of the exit part to be contiguous with the exit part, and supported by the housing. The exit part is exposed to the processing passage.