A sub-system 1 comprising: a sub-tank 2 for storing primary pure water W; a pump 4 provided at a base end portion of a supply line 3 for the primary pure water W stored in the tank 2; a heat exchanger 5 provided in the subsequent stage of the pump 4; a low-pressure ultraviolet (UV) irradiation oxidation device 6; a non-regenerative mixed-bed ion exchange device 7; and an ultrafiltration membrane (UF membrane) separation device 8 as a fine particle removal membrane device. In addition, a fine particle counter 9 which is a means for counting the number of fine particles is provided so as to be switchable between the outlet side of the UF membrane separation device 8 and the outlet side of the non-regenerative mixed-bed ion exchange device 7. This fine particle management method for an ultrapure water production system can maintain the number of fine particles in ultrapure water to be reduced.