An interface structure includes a sleeve, a wedge and a biasing member. The sleeve is arranged between the inner circumferential surface of the outer tube and the outer circumferential surface of the inner tube. The sleeve includes at least one fixing portion fixed to a first tube that is one of the outer tube and the inner tube, at least one contact surface configured to come into contact with a second tube that is the other of the outer tube and the inner tube, and a wedge mating surface configured to be mated with the wedge. The wedge is arranged between the first tube and the sleeve and is mated with the wedge mating surface of the sleeve. The biasing member biases the wedge so as to engage the wedge between the first tube and the sleeve while allowing the sleeve and the second tube to slide on each other.