TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The oxygen-containing gas is oxygen gas or cleaned dry air. Preferred Method: The chemical liquid preparation method of preparing a chemical liquid for substrate processing involves: a measurement step of measuring a dissolved oxygen concentration in the TMAH-containing chemical liquid; a nitrogen dissolution step of supplying a nitrogen-containing gas containing nitrogen gas to the TMAH-containing chemical liquid to make the nitrogen-containing gas dissolve in the TMAH-containing chemical liquid when the dissolved oxygen concentration measured in the measurement step is higher than a predetermined concentration; and an oxygen dissolution step of supplying the oxygen-containing gas to the TMAH-containing chemical liquid to make the oxygen-containing gas dissolve in the TMAH-containing chemical liquid when the dissolved oxygen concentration measured in the measurement step is lower than the predetermined concentration.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Unit: The chemical liquid preparation unit preparing a chemical liquid for substrate processing further comprises a measurement unit which measures a dissolved oxygen concentration in the TMAH-containing chemical liquid; a nitrogen dissolution unit which supplies a nitrogen-containing gas containing nitrogen gas to the TMAH-containing chemical liquid to make the nitrogen-containing gas dissolve in the TMAH-containing chemical liquid; and a controller which controls the nitrogen dissolution unit to make the nitrogen-containing gas dissolve in the TMAH-containing chemical liquid when the dissolved oxygen concentration measured by the measurement unit is higher than a predetermined concentration and controls the oxygen dissolution unit to make the oxygen-containing gas dissolve in the TMAH-containing chemical liquid when the dissolved oxygen concentration measured by the measurement unit is lower than the predetermined concentration. EXAMPLE - No suitable example given.