To provide an ultraviolet (UV) sterilizer which permits the easy maintenance, inspection and exchange of a UV lamp and can perform sterilization without being affected by the shades of sterilized microorganisms.

At least part of a sterilizer body 21 in which a liquid to be sterilized flows is composed of a UV transmitting wall 23 allowing the transmission of UV rays, and a flash lamp 24 for irradiation with UV rays is provided in the outer side of the wall 23. At least one guide plate 29 is disposed in the sterilizer body 21 for reversing the flow of the liquid to be sterilized to a direction intersecting with the flash radiated from the flash lamp 24. The guide plate 29 is composed of a UV transmitting material allowing the transmission of the UV rays. As a result, the flash lamp 34 can be provided on the outer side of the sterilizer body 21 formed of the wall 23 such that the maintenance, inspection, exchange, etc., of the UV lamp are facilitated and that the sterilization by uniform irradiation over the entire part is made possible in a manner as not to be shaded by the guide plates 29 of the UV transmitting material to reverse the flow.