A fluid sterilization device 10 includes: a flow passage tube 20 having a first end 21 and a second end 22 opposite to the first end 21 and defining a processing passage 12 extending in an axial direction； a light source (a first light source 50) provided in a vicinity of the first end 21 and radiating ultraviolet light from the first end 21 toward the processing passage 12 in the axial direction； and a housing (a first housing 30) that defines a straightening chamber (a first straightening chamber 13) encircling the first end 21 from outside in a radial direction. The straightening chamber includes a communication port (an outflow port 37) that serves as an inlet or an outlet for a fluid flowing in the processing passage 12, and communicates with the processing passage 12 via a gap between the first end 21 and an opposing member (a first window 36) opposing the first end 21 in the axial direction.
