A sterilization device includes a housing having a flow passage R of a fluid that is to be sterilized; and an LED element being disposed in the housing to irradiate ultraviolet light into the flow passage R, wherein an inner wall surface of the flow passage has a pair of reflection surfaces (a first reflection surface and a second reflection surface) which directs the ultraviolet light irradiated from the LED element from one side of the flow passage R to another side thereof while reflecting the ultraviolet light a plurality of times; and a return surface that includes a surface that is perpendicular to an optical axis of the light which has been reflected a plurality of times from one side to the other side, and returns the light toward its original direction is formed on a side of the first reflection surface.