The present invention provides a photocatalyst injection system 1 comprising: a reactor primary system coolant collection section 10 collecting a reactor primary system coolant containing a noble metal or noble metal ion from a reactor primary system 5; a photocatalyst addition section 20 adding a photocatalyst to the collected reactor primary system coolant; an ultraviolet irradiation section 30 irradiating, with ultraviolet rays 81, the coolant 72 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 40 injecting the coolant containing the noble metal-carrying photocatalyst into the reactor primary system 5.