To provide a method for controlling a fluid in a fluid device having a converging channel for converging at least two fluids, which does not limit any interface position of the fluids and can form a stable interface.

The method for controlling the fluid, which forms a multilayer flow at the converging channel by using the fluid device having the converging channel for converging at least two fluids, includes: the first step of introducing a first fluid and a second fluid to the converging channel and forming the multilayer flow with the interface of the first and second fluids at a flow rate at which the fluids do not make each other discontinuous; and the second step of adjusting a cross-section ratio of the first and second fluids at the converging channel by varying at least one of flow rates of the first and second fluids after completing the first step.