Provided is a fluid sterilization module capable of suppressing variation in ultraviolet light irradiation dose caused by a high flow velocity portion formed in a fluid flowing through a flow path due to an assembly error. To this end, a fluid sterilization module (1) includes an inner cylinder (21) forming a cylindrical processing flow path (21d) extending in a longitudinal direction, a first chamber (26) communicating with the processing flow path (21d) via an opening on one end side of the inner cylinder (21), an inflow portion (4) allowing an object to flow in the first chamber (26), an outflow portion (5) allowing the object having passed through the processing flow path (21d) to flow out from an other end side of the inner cylinder (21), and a light emitting element (34a) provided facing an opening on the other end side of the processing flow path (21d) and configured to irradiate the object passing through the processing flow path (21d) with ultraviolet light along the longitudinal direction. The first chamber (26) has an inner volume equal to or more than 2/3 of the cube of an equivalent inner diameter of the processing flow path (21d) and equal to or less than 3 times of inner volume of the processing flow path (21d).
