An annular seal member is provided to seal a tube joint between a tube opening and an inserted tube end. A valve portion of the seal member has a first valve extending from a heel portion, a second valve extending further toward the inserted tube end and a reduced portion provided at a boundary between the first and the second valve. The first valve abuts the inner periphery of the tube opening while the second valve abuts the outer periphery of the inserted tube end. In an uninstalled state, the second valve tapers from the first valve toward the center of the tube end. The inner diameter of the second valve is smaller than the outer diameter of the inserted tube end. Due to elastic deformation of the reduced portion, the second valve is capable of radial deformation.