A coupling structure includes a first coupling portion formed by one end portion of a first tube, and a second coupling portion formed by one end portion of a second tube, and is configured such that the first coupling portion is inserted in the second coupling portion, the first coupling portion and the second coupling portion are coupled, with the second tube being pivotable with respect to the first tube, and a gap is formed between an outer surface of the first coupling portion and an inner surface of the second coupling portion in a first state in which a fluid is not flowing through the flow path, and in a second state in which the fluid is flowing through the flow path, pressure of the fluid causes the first coupling portion to deform to expand toward an outer side of the flow path to reduce the gap.