An ultraviolet irradiation system includes: an ultraviolet irradiation apparatus including a plurality of ultraviolet lamps; a flowmeter configured to measure a flow rate of the water to be treated that passes through the ultraviolet irradiation apparatus; and an ultraviolet-dose monitoring and controlling apparatus configured to monitor an ultraviolet dose of the ultraviolet irradiation apparatus and to control outputs of the ultraviolet lamps. The plurality of ultraviolet lamps include a first ultraviolet lamp and a plurality of second ultraviolet lamps. The ultraviolet irradiation apparatus includes: a first measurement head configured to measure an ultraviolet intensity of the first ultraviolet lamp; and a plurality of second measurement heads configured to respectively measure ultraviolet intensities of the plurality of the ultraviolet lamps. A distance between the first ultraviolet lamp and the first measurement head is set to a determined value.