On March 31, 2022, the Ministry of Health, Labor and Welfare Health Bureau Notification “Establishment of Ministerial Ordinance Concerning Water Quality Standards and Partial Revision of the Ordinance for Enforcement of the Water Supply Act” (Notification No. 1010004 of Health Bureau, Ministry of Health, Labor and Welfare dated October 10, 2003) was partially revised. In the revision, it was stated that “Methidathione oxone, which is the oxone form of methidathione, should also be measured.”, it has been added as a component to be measured in Attachment 20-2 “Liquid Chromatograph - Simultaneous Analysis Using a Mass Spectrometer (LC-MS/MS Method)”. This time, we used InertSustain C18, which has a high degree of inertness, as the analytical column, and sodium ascorbate and sodium thiosulfate as dechlorinating agents for tap water.