The present disclosure provides a formation method for a slag flow that includes combining a first liquid and a second liquid that is not compatible with the first liquid, moving the second liquid that is combined with the first liquid to an upstream side with respect to a combination point of the first liquid and the second liquid in a flow direction of the first liquid, and moving, by the first liquid, the second liquid that is retained on the upstream side with respect to the combination point of the first liquid and the second liquid in the flow direction of the first liquid, to a downstream side with respect to the combination point of the first liquid and the second liquid in the flow direction of the first liquid, where a density D1 of the first liquid and a density D2 of the second liquid satisfy a relationship of D1>D2, and provides an application thereof.