The present invention addresses the problem of providing a fluid processing method with which processing properties of fluids to be processed can be effectively controlled. The present invention is provided with processing surfaces (1, 2) which are capable of being brought closer to each other and being separated from each other, and which rotate relatively. A fluid to be processed is made to pass from inside to outside in a processing area between the processing surfaces (1, 2) to obtain a fluid thin film, and the resultant fluid thin film of the fluid to be processed is subjected to processing. Processing properties are controlled by changing the ratio (Od/ Cd) of the distance (Od) to an outer peripheral end from a centre of rotation.