PROBLEM TO BE SOLVED: To provide a new technique for accurately detecting output fluctuation of a light source included in a fluid sterilizer. SOLUTION: A fluid sterilizer 10 has a tubular housing 14 having a processing flow path 12 in which a passing fluid is sterilized, a light source 16 that irradiates ultraviolet rays toward the processing flow path, and a light source and a processing flow. A window member 18 provided between the road and having an incident surface 18a on which ultraviolet rays emitted from a light source are incident and an emitting surface 18b on which ultraviolet rays are emitted toward a processing flow path, and ultraviolet rays emitted from a light source. Of these, a light receiving unit 20 that receives a part of the ultraviolet rays reflected by the incident surface is provided. The light source 16 is arranged so that the irradiated ultraviolet rays are obliquely incident on the incident surface, and the window member 18 has a housing in which the ultraviolet rays obliquely incident on the incident surface are refracted and emitted from the exit surface. It is configured to approach parallel to the axial direction Ax of. [Selection diagram] Fig. 1【課題】流体殺菌装置が備える光源の出力変動を精度良く検出する新たな技術を提供する。【解決手段】流体殺菌装置１０は、通過する流体が殺菌処理される処理流路１２を有する筒状の筐体１４と、処理流路に向けて紫外線を照射する光源１６と、光源と処理流路との間に設けられ、光源から照射された紫外線が入射する入射面１８ａと、処理流路に向けて紫外線が出射する出射面１８ｂと、を有する窓部材１８と、光源から照射された紫外線のうち入射面で反射された一部の紫外線を受光する受光部２０と、を備える。光源１６は、照射した紫外線が入射面に対して斜めに入射するように配置されており、窓部材１８は、入射面に斜めに入射した紫外線が屈折して出射面から出射する方向が筐体の軸方向Ａｘと平行に近づくように構成されている。【選択図】図１
