PROBLEM TO BE SOLVED: To monitor the operation of a light emitting element while continuously sterilizing a fluid. A fluid sterilizer 10 includes a straight tube 20 through which a fluid flows, and a light source 30 including a plurality of light emitting elements 32 that emit ultraviolet light from a first end 21 of the straight tube 20 toward the inside of the straight tube. , An imaging device 36 that images visible light output from each of the plurality of light emitting elements 32 and transmitted through the inside of the straight tube 20 at a second end portion 22 opposite to the first end portion 21 of the straight tube 20. Prepare A first window member 37 arranged between the light source 30 and the first end portion 21, and a first window member 37 arranged between the imaging device 36 and the second end portion 22 and having a lower ultraviolet light transmittance than the first window member 37. The second window member 38 may be further provided. [Selection diagram] Figure 1【課題】流体に対して連続的に殺菌処理をしながら発光素子の動作をモニタする。【解決手段】流体殺菌装置１０は、流体が流れる直管２０と、直管２０の第１端部２１から直管の内部に向けて紫外光を照射する複数の発光素子３２を含む光源３０と、複数の発光素子３２のそれぞれから出力されて直管２０の内部を透過する可視光を直管２０の第１端部２１とは反対の第２端部２２において撮像する撮像装置３６と、を備える。光源３０と第１端部２１の間に配置される第１窓部材３７と、撮像装置３６と第２端部２２の間に配置され、第１窓部材３７よりも紫外光の透過率が低い第２窓部材３８とをさらに備えてもよい。【選択図】図１
