To provide a device and method for efficiently removing a substance from a liquid or adding a substance to the liquid.

A device and method mix a liquid and a fluid by causing them pass through a porous medium, facilitate contact between the liquid and the fluid, and facilitate transfer of a component between the liquid and the fluid. For example, the device and method include a process of passing the liquid such as fuel and the fluid such as nonreactive gas through the porous medium, and can efficiently carry out treatment such as removal of a mixed substance and addition of an auxiliary substance to the liquid, by means of transferring a component such as oxygen gas or the like contained in the liquid into the fluid.