A chemical liquid preparation method of preparing a chemical liquid for treating a film formed on a substrate, including a gas dissolving process in which an oxygen-containing gas and an inert-gas-containing gas are dissolved in the chemical liquid by supplying the oxygen-containing gas which contains oxygen gas and the inert-gas-containing gas which contains an inert gas to a chemical liquid, wherein in the gas dissolving process, a dissolved oxygen concentration in the chemical liquid is adjusted by setting a mixing ratio between the oxygen-containing gas and the inert-gas-containing gas supplied to the chemical liquid as a mixing ratio corresponding to a predetermined target dissolved oxygen concentration.