To provide a method for washing an ultrapure water production system which can reduce washing time, and further can recycle used washing water as raw water.

In an ultrapure water production system 100 including: an ultrapure water production device 11; a water supply system of supplying ultrapure water produced by the ultrapure water production device 11 to its use point P; a return system of returning the used ultrapure water used in the use point P to the ultrapure water production device 11 through a recovery water tank 19; a disinfectant addition means of adding a disinfectant to the used ultrapure water flowing through the inside of the return system toward the recovery water tank 19; and a disinfectant removal means 23 of removing the added disinfectant in a space between the recovery water tank 19 and the ultrapure water production device 11, washing water obtained by adding the disinfectant to the ultrapure water in the water supply system is passed to the water supply system and the water return system for 2 to 10 hr to perform washing.