A photocatalyst injection system including: a reactor primary system coolant collection section collecting a reactor primary system coolant containing a noble metal or noble metal ion from a reactor primary system; a photocatalyst addition section adding a photocatalyst to the collected reactor primary system coolant; an ultraviolet irradiation section irradiating, with ultraviolet rays, the coolant to which the photocatalyst has been added for producing, in the coolant, a noble metal-carrying photocatalyst in which the noble metal is carried on a surface of the photocatalyst; and a noble metal-carrying photocatalyst injection section injecting the coolant containing the noble metal-carrying photocatalyst into the reactor primary system.