Provided is a method for producing an oxide and/or hydroxide wherein the ratio of oxide and hydroxide has been controlled. The method produces an oxide, a hydroxide, or a mixture thereof, and obtains an oxide and/or a hydroxide wherein the ratio of oxide and hydroxide has been controlled by means of changing a specific condition relating to at least one fluid to be processed introduced between processing surfaces (1, 2) when causing the precipitation of the oxide, hydroxide, or mixture thereof by mixing an basic fluid containing at least one type of basic substance and a fluid containing at least one type of metal or metallic substance as the fluids to be processed between the processing surfaces (1, 2) that are provided facing each other, are able to approach to and separate from each other, and of which at least one rotates relative to the other. The specific condition is at least one condition selected from the group consisting of: the speed of introduction of at least one of the fluids to be processed; and the pH of at least one of the fluids to be processed.