An object of the present invention is to provide an ultraviolet sterilizer capable of detecting breakage of an ultraviolet transmitting plate of, e.g., quartz glass with high accuracy while suppressing attenuation of ultraviolet light by the ultraviolet transmitting plate. Provided is an ultraviolet sterilizer comprising: an ultraviolet lamp a housing surrounding the ultraviolet lamp an ultraviolet transmitting section provided at an opening portion on a lower side of the housing, the ultraviolet transmitting section including an ultraviolet transmitting plate and an ultraviolet transmitting plate-holding means and an air flow detecting means, wherein the ultraviolet transmitting section-holding means comprises a sealing means for forming an enclosed space which one end is in communication with the air flow detecting means, cooperating with an outer peripheral part of the ultraviolet transmitting plate, the air flow detecting means comprises an air pressure adjusting-means for adjusting an air pressure inside the enclosed space, and a flowmeter, and wherein a flow of air inside the enclosed space generated when the enclosed space is released from a sealed state due to breakage of the ultraviolet transmitting plate, is detected by the flowmeter.
