Abstract: According to one embodiment, an apparatus for measuring hydroxyl radicals that measures hydroxyl radicals produced by irradiating a liquid to be treated flowing through a channel in which an ultraviolet lamp is arranged with ultraviolet rays, the apparatus includes a diverting unit, a reagent adding unit, and a measuring unit. The diverting unit has a diverting channel that diverts the liquid to be treated before being irradiated with the ultraviolet rays from the channel and part of which is arranged at a position enabling the liquid to be treated within the channel to be irradiated with the ultraviolet rays. The reagent adding unit adds a hydroxyl radical measuring reagent to the diverted liquid to be treated. The measuring unit irradiates the diverted liquid to be treated with the ultraviolet rays and measures the amount of hydroxyl radicals produced based on a change in the hydroxyl radical measuring reagent between before and after the irradiation with the ultraviolet rays.