A method is provided to collect liquid, such as pancreatic juice water, and saline, from a body cavity of a subject. In the method, a tool is inserted through the body cavity to reach a target site in the body cavity. A body position of the subject is adjusted such that the wall surface of the body cavity including the target site intersects with a direction of gravitational force and the liquid present in the body cavity gathers on the wall surface intersecting with the direction of gravitational force. A recess is formed to be concaved on the wall surface in the direction of the gravitational force by making the tool press the wall surface of the body cavity. Therefore, the liquid present or discharged in the body cavity is accumulated in the recess. The liquid accumulated in the recess is taken out of the body of the subject.