A fluid sterilization device includes: a flow passage tube in which a processing passage where a passing fluid is sterilized is formed; an inflow passage or an outflow passage formed in the flow passage tube; a light source that irradiates the processing passage with ultraviolet light; and a rotating body provided in the processing passage. The rotating body is rotated around a longitudinal direction of the processing passage in response to a flow of the fluid passing through the processing passage and is configured to come into contact with an inner wall of the flow passage tube during rotation.