This invention aims to suppress temperature rise of a light source and increase irradiation efficiency of ultraviolet rays to a fluid flowing through a flow path member. The fluid sterilizing apparatus of the embodiment includes a flow path member having a first flow path for flowing a fluid in a first direction, a second flow path communicating with the first flow path and returning the fluid from the first direction to a second direction, and a light source, which is disposed to face the cross-section of the first and second flow paths and the cross-section intersecting the first and second directions, irradiates ultraviolet rays into the first flow path and the second flow path.本發明抑制光源的溫度上升，並且提高對在流路構件中流動的流體的紫外線照射效率。實施方式的流體殺菌裝置包括：流路構件，具有第1流路和第2流路，所述第1流路用於使流體朝第1方向流動，所述第2流路連通於第1流路，且用於使流體朝從第1方向返回的第2方向流動；以及光源，與作為第1流路及第2流路的流路剖面且與第1方向及第2方向交叉的流路剖面相向地配置，朝第1流路內及第2流路內照射紫外線。
