To heighten measurement accuracy when an indicator reacting with oxygen is injected into a measuring cell storing a liquid to be measured filtered by using a filter and the concentration of dissolved oxygen included in the liquid to be measured is measured based on the change occurring in the liquid to be measured at that time.

This measuring method of the dissolved oxygen concentration includes a storage process for supplying the measuring cell 20 with the liquid to be measured through a sample introduction route 26 having the replaceable filter 27 and storing it, an injection process for injecting the indicator into the liquid to be measured stored in the measuring cell 20, and a determination process for determining the change generated in the liquid to be measured by the indicator. When the filter is replaced a purging processing for purging air from the filter 27 is performed before the storage process.