A method for sterilizing a liquid including the steps of: making a liquid to be sterilized which is a solution or a suspension flow in a flow passage formed between a pair of walls arranged in parallel with a gap therebetween； irradiating an ultraviolet ray from one or two ultraviolet ray emitting surface arranged on one or both of opposite two faces of the pair of walls to the liquid flowing through the flow passage, wherein a width of the gap is no greater than a sum of effective optical path length of the one or two ultraviolet ray emitting surface； and the effective optical path length of the ultraviolet ray emitting surface is defined as a thickness of a layer of the liquid such that irradiance of a transmitted ultraviolet ray which has been emitted from the ultraviolet ray emitting surface and have passed through the layer becomes 0.001mW/cm2.本發明中之液體殺菌方法，係包含：(1)使作為溶液或是懸濁液的被殺菌液體流通於流路的步驟，該流路為設有預定寬幅之間隙地形成在配置成平行的一對區隔壁之間、以及(2)從在該一對區隔壁之相對向的2面之一方或雙方所設置的1或2個紫外線發光面，對通過該流路的被殺菌液體照射紫外線的步驟，上述間隙之與紫外線發光面垂直之方向上的寬幅，為上述1或2個紫外線發光面的有效光路徑長度的總和以下，而紫外線發光面的有效光路徑長度，是定義為：從該紫外線發光面所照射出來的紫外線，在穿透被殺菌液體之層後之穿透紫外線的輻射照度成為0.001mW/cm2時之上述被殺菌液體之層的厚度。
