The invention provides a valve structure. The valve member 38 of a valve structure 32 is rotatably supported from an outer tube 34 by a rotation shaft 40. The rotation shaft 40 is fixed to the valve member 38 at a position inside a tangent to the outer periphery of the valve member 38 and avoiding the center point. A return spring 52 is a tension spring that causes tension force to constantly act on a link arm 54 to generate a rotation force toward the closing direction of the valve member 38.