A method of setting a distance between a first ultraviolet lamp and a first measurement head of an ultraviolet irradiation system comprising an ultraviolet irradiation apparatus including a plurality of ultraviolet lamps (13-1,..., 13-6), which include said first ultraviolet lamp and a plurality of second ultraviolet lamps (13-2,..., 13-6), and said measurement head said measurement head being separated from said first ultraviolet lamp by said distance, and said ultraviolet irradiation apparatus being configured to emit ultraviolet rays onto water to be treated in the ultraviolet irradiation apparatus and to discharge the water to be treated after ultraviolet irradiation, said method comprising: measuring, by a flowmeter of said ultraviolet irradiation system, a flow rate of the water to be treated that passes through the ultraviolet irradiation apparatus; and monitoring, by an ultraviolet-dose monitoring and controlling apparatus of said ultraviolet irradiation system, an ultraviolet dose of the ultraviolet irradiation apparatus, and controlling outputs of said first ultraviolet lamp, measuring, by said first measurement head of said ultraviolet irradiation system, an ultraviolet intensity of the first ultraviolet lamp; and measuring, by a plurality of second measurement heads (25-1,..., 25-6), respective ultraviolet intensities of the plurality of the ultraviolet lamps (13-1,..., 13-6), and setting a distance between the first ultraviolet lamp (13 - 1) and the first measurement head such that a single linear relationship is formed between a reduction equivalent ultraviolet dose and a ratio (S/S0) of the ultraviolet intensity measured by the first measurement head to the ultraviolet intensity pre-measured by the first measurement head when the water to be treated has an ultraviolet transmissivity of 100% and an output control value of the first ultraviolet lamp is 100%, that is when an electrical power is inputted in the first ultraviolet lamp that is equal to the rated electric power of the first ultraviolet lamp, said single linear relationship being irrespective of the ultraviolet transmissivity of the water to be treated. measuring, by a flowmeter of said ultraviolet irradiation system, a flow rate of the water to be treated that passes through the ultraviolet irradiation apparatus; and monitoring, by an ultraviolet-dose monitoring and controlling apparatus of said ultraviolet irradiation system, an ultraviolet dose of the ultraviolet irradiation apparatus, and controlling outputs of said first ultraviolet lamp, measuring, by said first measurement head of said ultraviolet irradiation system, an ultraviolet intensity of the first ultraviolet lamp; and measuring, by a plurality of second measurement heads (25-1,..., 25-6), respective ultraviolet intensities of the plurality of the ultraviolet lamps (13-1,..., 13-6), and setting a distance between the first ultraviolet lamp (13 - 1) and the first measurement head such that a single linear relationship is formed between a reduction equivalent ultraviolet dose and a ratio (S/S0) of the ultraviolet intensity measured by the first measurement head to the ultraviolet intensity pre-measured by the first measurement head when the water to be treated has an ultraviolet transmissivity of 100% and an output control value of the first ultraviolet lamp is 100%, that is when an electrical power is inputted in the first ultraviolet lamp that is equal to the rated electric power of the first ultraviolet lamp, said single linear relationship being irrespective of the ultraviolet transmissivity of the water to be treated.