Abstract: A cleaning solution preparation device includes a deionized water supply source, a gas supply source, a gas-dissolving unit, and a gas supply pressure controller. The gas supply source supplies any of an oxidative gas, a reductive gas, an inert gas, a mixed gas of an oxidative gas and an inert gas, or a mixed gas of a reductive gas and an inert gas. The gas-dissolving unit dissolves the gas supplied from the gas supply source in deionized water supplied from the deionized water supply source to supply a gas-dissolved cleaning solution to objects to be cleaned. The gas supply pressure controller controls the pressure of the supplied gas at a value exceeding the atmospheric pressure when dissolving the gas in the deionized water.