The present invention has an object of providing a fluid sterilization device that can effectively use wide-angle light of ultraviolet light emitted from a light source unit and allows a fluid to stay in an irradiation range of the ultraviolet light even with a quite simple structure. The fluid sterilization device according to the present invention includes a first flow path portion that extends in an axial direction and allows a fluid to pass from a first end to a second end, a light source unit that is connected to the second end of the first flow path portion and irradiates the fluid with ultraviolet light, a second flow path portion that is opposed to the first flow path portion with the light source unit interposed between the first flow path portion and the second flow path portion, and a third flow path portion that is arranged outside of the light source unit in a radial direction and is configured to allow the fluid flowing through the first flow path portion to flow into the second flow path portion. In the fluid sterilization device, the first flow path portion or the light source unit includes a first communication portion that communicates with the third flow path portion, the second flow path portion or the light source unit includes a second communication portion that communicates with the third flow path portion, and the third flow path portion is any of polymers including polytetrafluoroethylene (PTFE), a perfluoroethylene propene copolymer (FEP), perfluoroalkoxyalkane (PFA), and polypropylene (PP). The first communication portion includes a cut-out communication region that is formed by cutting out a part of a connection portion between the first flow path portion and the light source unit.