PROBLEM TO BE SOLVED: To provide a fluid sterilizer having enhanced accuracy of monitoring.SOLUTION: A fluid sterilizer 10 includes: a straight tube 12 which partitions a treatment passage 28； a light source 40 which irradiates ultraviolet light in an axial direction of the straight tube 12 toward the treatment passage 28； and a light-receiving section 46 which receives a part of light of the ultraviolet light output from the light source 40. The straight tube 12 has a recess 18 which is formed of a fluorine-based resin and is formed on a part of an outer wall surface 17 of the straight tube 12 so that a thin portion 19 having a thickness tin a radial direction from an inner wall surface 16 of the straight tube 12 thinner than a thickness tof the other portion is partially provided. The light-receiving section 46 is arranged so as to receive ultraviolet light having permeated through the thin portion 19 in the recess 18.SELECTED DRAWING: Figure 1【課題】モニタリングの精度を高めた流体殺菌装置を提供する。【解決手段】流体殺菌装置１０は、処理流路２８を区画する直管１２と、処理流路２８に向けて直管１２の軸方向に紫外光を照射する光源４０と、光源４０から出力される紫外光の一部を受光する受光部４６と、を備える。直管１２は、フッ素系樹脂で構成され、かつ、直管１２の内壁面１６からの径方向の厚さｔ２が他の部分の厚さｔ１より小さい肉薄部１９が部分的に設けられるよう直管１２の外壁面１７の一部に形成される凹部１８を有する。受光部４６は、凹部１８にて肉薄部１９を透過した紫外光を受光するよう配置される。【選択図】図１
