A new technique for bringing a flow in a flow path close to a desired state is provided. A fluid sterilizer 20 includes a channel tube 22 in which a processing channel 36 in which a fluid passing therethrough is sterilized, a first light source 28 that irradiates ultraviolet rays toward the processing channel 36, An inflow path 42b formed in a direction intersecting with the outer peripheral surface 22c of the flow path tube 22 and a communication path 78 that connects the inflow path 42b and the processing flow path 36 are provided. The communication path 78 has a narrow path 80 that is narrower than the inflow path side on the way from the inflow path 42 b to the opening 22 d of the first end portion 32 of the flow path pipe 22. [Selection] Figure 5【課題】流路内の流れを所望の状態に近づける新たな技術を提供する。【解決手段】流体殺菌装置２０は、通過する流体が殺菌処理される処理流路３６が形成されている流路管２２と、処理流路３６に向かって紫外線を照射する第１光源２８と、流路管２２の外周面２２ｃと交差する向きに形成された流入路４２ｂと、流入路４２ｂと処理流路３６とを連通する連通路７８と、を備える。連通路７８は、流入路４２ｂから流路管２２の第１端部３２の開口２２ｄへ向かう途中に、流入路側よりも狭くなった狭路８０を有する。【選択図】図５
