The present invention provides a chemical-liquid mixing method and a chemical-liquid mixing apparatus capable of sufficiently generating a peroxomonosulfuric acid that is effective in removing a resist from a substrate, when a sulfuric acid and a hydrogen peroxide solution are mixed with each other. At first, an inner tank 10 is filled up with a sulfuric acid and the sulfuric acid overflowing from the inner tank 10 is allowed to flow into an outer tank 12. Then, a hydrogen peroxide solution is supplied into the inner tank 10 and the hydrogen peroxide solution is allowed to flow into the outer tank 12 whereby the two kinds of liquids of the hydrogen peroxide solution and the sulfuric acid are stored in the outer tank 12. Simultaneously when the hydrogen peroxide solution flows into the outer tank 12, a return pump 16 is activated.