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